Lens barrel

ABSTRACT

A lens barrel includes an imaging optical system having front and rear optical elements with a spring installed therebetween; a reference barrel which includes a forward-facing limit surface and a rearward-facing limit surface; a first position control mechanism which includes a first rotational member and a first limit member, and varies a position of the front optical element in the optical axis direction relative to the reference barrel; and a second position control mechanism which includes a second rotational member and a second limit member, and varies a position of the rear optical element in the optical axis direction relative to the reference barrel. The first limit member is brought into contact with the rearward-facing limit surface by a forward biasing force of the spring while the second limit member is brought into contact with the forward-facing limit surface by a rearward biasing force of the spring.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a lens barrel, and in particular,relates to a backlash eliminating structure of a lens barrel includingtwo optical elements one of which is positioned in front of the other.

2. Description of the Related Art

In photographing lenses (photographing lens barrels) such as zoom lenseswhich include one or more optical elements that move in an optical axisdirection when a focal-length varying operation (zooming operation) isperformed, a backlash eliminating structure for movable members whichmove in the optical axis direction is essential. This type of backlasheliminating structure (e.g., disclosed in Japanese Unexamined PatentPublication No. 2004-170961, Japanese Unexamined Patent Publication No.2006-337563) requires many springs and thus tends to be complicated instructure.

SUMMARY OF THE INVENTION

The present invention provides a simple backlash eliminating structureof a lens barrel which includes two movable optical elements, i.e.,front and rear optical elements movable relative to each other in anoptical axis direction and a drive mechanism for these two movableoptical elements.

The present invention provides a lens barrel in which backlash betweenelements associated with the front and rear optical elements can beeliminated simply by a spring installed between the front and rearoptical elements.

According to an aspect of the present invention, a lens barrel isprovided, including an imaging optical system which includes a frontoptical element and a rear optical element which are relatively movablein an optical axis direction; a spring installed between the frontoptical element and the rear optical element so as to increase adistance therebetween; a reference barrel which includes aforward-facing limit surface and a rearward-facing limit surface whichface forward and rearward in the optical axis direction, respectively; afirst position control mechanism which includes a first rotationalmember and a first limit member, the first position control mechanismvarying a position of the front optical element in the optical axisdirection relative to the reference barrel in accordance with a rotationof the first rotational member; and a second position control mechanismwhich includes a second rotational member and a second limit member, thesecond position control mechanism varying a position of the rear opticalelement in the optical axis direction relative to the reference barrelin accordance with a rotation of the second rotational member. The firstlimit member is brought into contact with the rearward-facing limitsurface by a forward biasing force of the spring to determine a positionof the first position control mechanism while the second limit member isbrought into contact with the forward-facing limit surface by a rearwardbiasing force of the spring to determine a position of the secondposition control mechanism.

It is desirable for the forward-facing limit surface to be formed at aposition closer to the front of the reference barrel in the optical axisdirection than the rearward-facing limit surface.

It is desirable for the first position control mechanism to include alinear guide ring serving as the first limit member, a position of whichin the optical axis direction is determined by the rearward-facing limitsurface of the reference barrel, wherein the linear guide ring isprevented from rotating relative to the reference barrel; and a camring, a position of which in the optical axis direction is determined bythe linear guide ring and which determines the position of the frontoptical element in the optical axis direction. It is desirable for thesecond position control mechanism to include a rotating ring includingthe second limit member, a position of which in the optical axisdirection is determined by the forward-facing limit surface of thereference barrel and which rotates relative to the reference barrel; andat least one cam groove, formed on the rotating ring, for determiningthe position of the rear optical element in the optical axis direction.

It is desirable for the rearward-facing limit surface of the referencebarrel to be provided at an end of a linear guide groove for guiding thelinear guide ring to move linearly in the optical axis direction, andfor the forward-facing limit surface of the reference barrel to beformed by a side wall of a circumferential groove for guiding therotating ring so as to allow the rotating ring to rotate.

It is desirable for the front optical element and the rear opticalelement to constitute two lens groups of a zoom optical system, and forthe two lens groups to be moved toward and away from each other in apredetermined moving manner in the optical axis direction by a rotationof the cam ring and the rotating ring when the zoom optical systemperforms a zooming operation.

It is desirable for each of the front optical element and the rearoptical element to be movable between a retracted position and aready-to-photograph position in the optical axis direction. The firstlimit member and the second limit member are in contact with therearward-facing limit surface and the forward-facing limit surface,respectively, when each of the front optical element and the rearoptical element is in the ready-to-photograph position. The first limitmember and the second limit member are disengaged from therearward-facing limit surface and the forward-facing limit surface,respectively, when each of the front optical element and the rearoptical element is in the retracted position.

The spring can be a coil spring.

It is desirable for the reference barrel to be a stationary member thatis fixed to a camera body to which the lens barrel is mounted.

In an embodiment, a lens barrel is provided, including an imagingoptical system which includes a front optical element and a rear opticalelement which are relatively movable in an optical axis direction; aspring installed between the front optical element and the rear opticalelement so as to increase a distance therebetween; a reference barrelwhich includes a forward-facing limit surface and a rearward-facinglimit surface which face forward and rearward in the optical axisdirection, respectively; a first limit member for determining a positionin the optical axis direction of the front optical element; a secondlimit member for determining a position in the optical axis direction ofthe rear optical element; a first cam mechanism which is providedbetween the front optical element and the first limit member forcontrolling a position in the optical axis direction of the frontoptical element, the first cam mechanism transmitting a forward biasingforce of the spring to the first limit member so as to bring the firstlimit member into contact with the rearward-facing limit surface; and asecond cam mechanism which is provided between the rear optical elementand the second limit member for controlling a position in the opticalaxis direction of the rear optical element, the second cam mechanismtransmitting a rearward biasing force of the spring to the second limitmember so as to bring the second limit member into contact with theforward-facing limit surface.

In an embodiment, a lens barrel is provided, including an imagingoptical system which includes a front optical element and a rear opticalelement which are relatively movable in an optical axis direction; aspring, installed between the front optical element and the rear opticalelement, for biasing the front optical element and the rear opticalelement in opposite directions one of toward and away from each other inthe optical axis direction; a reference barrel which includes aforward-facing limit surface and a rearward-facing limit surface whichface forwardly and rearwardly in the optical axis direction,respectively; a first position control mechanism which includes a firstrotational member and a first limit member, the first position controlmechanism varying a position of the front optical element in the opticalaxis direction relative to the reference barrel in accordance with arotation of the first rotational member; a second position controlmechanism which includes a second rotational member and a second limitmember, the second position control mechanism varying a position of therear optical element in the optical axis direction relative to thereference barrel in accordance with a rotation of the second rotationalmember. The first limit member is brought into contact with one of theforward-facing limit surface and the rearward-facing limit surface whichfaces toward a direction opposite to a direction of a biasing forceapplied on the front optical element by the spring, and the second limitmember is brought into contact with the other of the forward-facinglimit surface and the rearward-facing limit surface, which faces towarda direction opposite to a direction of a biasing force applied on therear optical element by the spring.

According to the present invention, backlash can be eliminated by asimple structure with fewer number of springs in a lens barrel whichincludes two (front and rear) movable optical elements and a drivemechanism for these two movable optical elements, wherein the twomovable optical elements are movable relative to each other in anoptical axis direction.

The present disclosure relates to subject matter contained in JapanesePatent Application No. 2008-65727 (filed on Mar. 14, 2008) which isexpressly incorporated herein by reference in its entirety.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described below in detail with referenceto the accompanying drawings in which:

FIG. 1 is a cross sectional view of an embodiment of a zoom lens barrelaccording to the present invention, showing the zoom lens barrel in theretracted state (fully-retracted state);

FIG. 2 is a cross sectional view of the zoom lens barrel set at thewide-angle extremity in a ready-to-photograph state, showing only anupper half of the zoom lens barrel from an imaging optical axis;

FIG. 3 is a cross sectional view of the zoom lens barrel set at thetelephoto extremity in a ready-to-photograph, showing only an upper halfof the zoom lens barrel from the imaging optical axis;

FIG. 4 is an exploded perspective view of a portion of the zoom lensbarrel;

FIG. 5 is an exploded perspective view of a portion of the zoom lensbarrel;

FIG. 6 is an exploded perspective view of another portion of the zoomlens barrel;

FIG. 7 is a developed plan view of a portion of a stationary barrel ofthe zoom lens barrel, wherein the structure of the inner peripheralsurface thereof is transparently shown (by broken lines) for claritypurposes;

FIG. 8 is a developed plan view of a helicoid ring of the zoom lensbarrel, wherein the structure of the inner peripheral surface thereof istransparently shown (by broken lines) for clarity purposes;

FIG. 9 is a developed plan view of a first linear guide ring of the zoomlens barrel;

FIG. 10 is a developed plan view of a cam ring of the zoom lens barrel;

FIG. 11 is a developed plan view of a frontmost external barrel of thezoom lens barrel;

FIG. 12 is a developed plan view of a first lens group holding ring ofthe zoom lens barrel;

FIG. 13 is a cross sectional view of the zoom lens barrel set at thewide-angle extremity in the ready-to-photograph state, showing theoperation of an inter-lens-group biasing spring provided between asecond lens group and a third lens group of the zoom lens barrel,showing only an upper half of the zoom lens barrel from the imagingoptical axis;

FIG. 14 is a front perspective view of an image sensor holding unit anda third lens group frame of the zoom lens barrel, showing the relativeposition between the image sensor holding unit and the third lens groupframe that is rotated to an off-axis displaced position by aposition-control cam bar which projects from the image sensor holdingunit;

FIG. 15 is a front perspective view of the image sensor holding unit andthe third lens group frame shown in FIG. 14, showing a state where thethird lens group frame held in the off-axis displaced position relativeto the image sensor holding unit has been retracted to the retractedposition;

FIG. 16 is a rear perspective view of a third lens group moving ring ofthe zoom lens barrel;

FIG. 17 is a rear perspective view of the third lens group frame and thecam ring, showing the positional relationship therebetween when thethird lens group frame is in an on-axis position;

FIG. 18 is a rear perspective view of the third lens group frame and thecam ring, showing the positional relationship therebetween when thethird lens group frame is in the off-axis displaced position;

FIG. 19 is a rear elevational view of the third lens group moving ringand the cam ring when the zoom lens barrel is set at the wide-angleextremity, viewed from the rear in the optical axis direction;

FIG. 20 is a rear elevational view of the third lens group moving ringand the cam ring after the cam ring is rotated in a retracting directionfrom the ready-to-photograph state of the zoom lens barrel set at thewide-angle extremity, viewed from the rear in the optical axisdirection;

FIG. 21 is a rear elevational view of the third lens group moving ringand the cam ring in a state where the third lens group frame is at amid-position between the on-axis position and the off-axis displacedposition, viewed from the rear in the optical axis direction;

FIG. 22 is a rear elevational view of the third lens group moving ringand the cam ring in the retracted state of the zoom lens barrel, viewedfrom the rear in the optical axis direction;

FIG. 23 is a developed plan view of a portion of the helicoid ring, aportion of the first linear guide ring and a guide roller of the camring, showing the positional relationship among a roller-engaging grooveof the helicoid ring, a roller-guiding cam slot of the first linearguide ring and the guide roller of the cam ring;

FIG. 24 is a view corresponding to that of FIG. 23 and illustrates astate where the guide roller of the cam ring moves in a circumferentialgroove of the roller-engaging groove in accordance with rotation of thehelicoid ring in a lens barrel advancing direction from the retractedstate of the zoom lens barrel;

FIG. 25 is a view corresponding to that of FIG. 23 and illustrates astate where the guide roller of the cam ring has reached the boundarybetween the circumferential groove and a rotational transfer grooveportion of the roller-engaging groove after the helicoid ring is furtherrotated in the lens barrel advancing direction;

FIG. 26 is a view corresponding to that of FIG. 23 and illustrates astate where the guide roller of the cam ring has entered the rotationaltransfer groove portion of the roller-engaging groove after the helicoidring is further rotated in the lens barrel advancing direction;

FIG. 27 is a view corresponding to that of FIG. 23 and illustrates thepositional relationship among the roller-engaging groove of the helicoidring, the roller-guiding cam slot of the first linear guide ring and theguide roller of the cam ring;

FIG. 28 is a rear perspective view of main elements of a lens barriermechanism and the cam ring that operates to control the barrieropening/shutting operation of the lens barrier mechanism;

FIG. 29 is a front elevational view of the elements of the lens barriermechanism shown in FIG. 28, showing a state where barrier blades of thelens barrier mechanism are shut;

FIG. 30 is a view corresponding to that of FIG. 29, showing a statewhere the barrier blades of the lens barrier mechanism are fully open;

FIG. 31 is a developed plan view of the cam ring, the frontmost externalbarrel and the first lens group holding ring, and illustrates a mannerof guiding lead projections of the frontmost external barrel andfirst-lens-group-control cam followers of the first lens group holdingring by first and second lead cam grooves formed on the cam ring;

FIG. 32 is a longitudinal cross sectional view of a portion of the zoomlens barrel and shows the relative position between the first lens groupand the lens barrier mechanism when the zoom lens barrel is in theretracted state, showing only an upper half of the zoom lens barrel fromthe imaging optical axis;

FIG. 33 is a view similar to that of FIG. 32 and shows the relativeposition between the first lens group and the lens barrier mechanismwhen the zoom lens barrel is in the ready-to-photograph state, showingonly an upper half of the zoom lens barrel from the imaging opticalaxis;

FIG. 34 is a graph showing the moving path of the first lens group inthe zooming range of the zoom lens barrel, and further showing the camtrack of each second lead cam groove and the cam track of eachroller-guiding cam slot, which achieves this moving path of the firstlens group;

FIG. 35 is a developed plan view of a portion of a helicoid ring, aportion of a linear guide ring and a guide roller of a cam ring in acomparative example of an idle mechanism for the cam ring; and

FIG. 36 is a view similar to that of FIG. 35, illustrating a state wherethe idle mechanism for the cam ring reaches a state corresponding to theretracted state of the zoom lens barrel.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of a zoom lens (zoom lens barrel) 5 according to thepresent invention is of a type which is incorporated in a digitalcamera. The zoom lens 5 is provided with an imaging optical system whichincludes a first lens group LG1, a second lens group (front opticalelement) LG2, a third lens group (rear optical element) LG3, a fourthlens group LG4, a low-pass filter (optical filter) LF and a solid-stateimage sensor (hereinafter referred to simply as image sensor) 60, inthat order from the object side in a ready-to-photograph state as shownin FIGS. 2 and 3. “Z1” shown in FIGS. 1 through 3 and other drawingsrepresents the imaging optical axis of the imaging optical system thatis configured as a zoom optical system. A zooming operation is carriedout by moving the first lens group LG1, the second lens group LG2 andthe third lens group LG3 along the imaging optical axis Z1 in apredetermined moving manner, and a focusing operation is carried out bymoving the fourth lens group LG4 along the same imaging optical axis Z1.In the following descriptions, the term “optical axis direction” refersto a direction along or parallel to the imaging optical axis Z1 of theimaging optical system unless otherwise stated. In addition, the terms“circumferential direction” and “rotational direction” refer to adirection about the axes (which are substantially coincident with theimaging optical axis Z1) of ring or annular members which serve aselements of the zoom lens barrel 5.

The zoom lens 5 is provided on the radially outermost side thereof witha stationary barrel (reference barrel) 22 fixed to a camera body towhich the zoom lens 5 is mounted, and is provided immediately behind thestationary barrel 22 with an image sensor holding unit 21 fixed to theback of the stationary barrel 22. The image sensor 60 is mounted on theimage sensor holding unit 21 and held thereby in a manner to be movablealong a plane parallel to the imaging optical axis Z1. The image sensorholding unit 21 has the capability of correcting (reducing) image shakeby moving the image sensor 60 along the plane orthogonal to the imagingoptical axis Z1.

The zoom lens 5 is provided in the stationary barrel 22 with an AF lensframe 51 which is guided linearly in the optical axis direction, i.e.,without rotating about the optical axis Z1, via an AF guide shaft 52.The AF lens frame 51 holds the fourth lens group LG4. As shown in FIG.13, the AF lens frame 51 is biased forward in the optical axis directionto abut against an AF nut 32 by a fourth-lens-group biasing spring 33 inthe form of a compression coil spring which is installed between the AFlens frame 51 and the image sensor holding unit 21. The AF nut 32 isscrew-engaged with a lead screw 31 while being prevented from rotating.The lead screw 31 is driven to rotate on the axis of rotation thereof byan AF motor 160 mounted to the stationary barrel 22. Accordingly,rotating the lead screw 31 forward and reverse by the AF motor 160causes the AF nut 32 to move forward and rearward along the lead screw31 without rotating with the lead screw 31, thus causing the position ofthe AF lens frame 51 in the optical axis direction to vary in accordancewith this movement of the AF nut 32.

The zoom lens 5 is provided with a zoom motor 150 for rotating the zoomgear 28. The zoom motor 150 is mounted to the stationary barrel 22. Thezoom lens 5 is provided immediately inside the stationary barrel 22 witha helicoid ring (second limit member/second rotational member/rotatingring/outer advancing barrel) 18 which advances from and retracts intothe stationary barrel 22. The zoom gear 28 is in mesh with an outercircumferential gear 18 c formed on an outer peripheral surface of thehelicoid ring 18. As shown in FIG. 7, the stationary barrel 22 isprovided on an inner peripheral surface thereof with an inner helicoid22 a which is inclined at a predetermined angle of inclination withrespect to the optical axis direction, and an annular circumferentialgroove 22 b which lies in a plane orthogonal to the imaging optical axisZ1, thus having no axial-direction component. An outer helicoid 18 aformed on an outer peripheral surface of the helicoid ring 18 is engagedwith the inner helicoid 22 a of the stationary barrel 22. The helicoidring 18 can move in the optical axis direction while rotating relativeto the inner helicoid 22 a while being guided by the outer helicoid 18 aand the inner helicoid 22 a. The helicoid ring 18 is provided on anouter peripheral surface thereof with a set of three guide projections(second limit member) 18 b. Upon the helicoid ring 18 advancing to apredetermined forward position, the set of three guide projections 18 benter the circumferential groove 22 b. Thereupon, the helicoid ring 18only rotates about the imaging optical axis Z1, i.e., without moving inthe optical axis direction relative to the stationary barrel 22. Thestationary barrel 22 is provided on an inner peripheral surface thereofwith a set of three lead grooves 22 c which are communicativelyconnected to the circumferential groove 22 b and extend parallel tothreads of the inner helicoid 22 a. The set of three guide projections18 b of the helicoid ring 18 remain engaged in the set of three leadgrooves 22 c, respectively, during the time the inner helicoid 22 a andthe outer helicoid 18 a are engaged with each other. Both sides of eachguide projection 18 b are formed as a pair of inclined surfaces parallelto the associated lead groove 22 c, and each guide projection 18 b ismovable along the associated lead groove 22 c with the pair of inclinedsurfaces being in sliding contact with the pair of opposed side walls ofthe associated lead groove 22 c. On the other hand, each of the frontand rear surfaces of each guide projection 18 b in the optical axisdirection is formed as a circumferential flat surface lying in a planeorthogonal to the imaging optical axis Z1, and these front and rearcircumferential flat surfaces of each guide projection 18 b areprevented from moving in the optical axis direction relative to theopposed side walls (front and rear side walls) of the circumferentialgroove 22 b when each guide projection 18 b is positioned in thecircumferential groove 22 b.

The zoom lens 5 is provided with a first linear guide ring (first limitmember) 14 which is positioned inside the helicoid ring 18 and supportedthereby. FIG. 9 is a developed plan view of the first linear guide ring14. The first linear guide ring 14 is provided with an annular flange 14a which projects radially outwards from the rear end of the first linearguide ring 14, and is provided on the annular flange 14 a with aplurality of linear guide projections (first limit member) 14 b whichproject radially outward from the annular flange 14 a. The first linearguide ring 14 is guided linearly in the optical axis direction relativeto the stationary barrel 22 via the engagement of the plurality oflinear guide projections 14 b with a plurality of linear guide groove 22d (only one of which appears in FIGS. 4 and 7) formed on an innerperipheral surface of the stationary barrel 22. As shown in FIG. 7, thefront end of each linear guide groove 22 d is closed to serve as a limitwall portion (rearward-facing limit surface) 22 e and each linear guidegroove 22 d is open at the rear end. In addition, the first linear guidering 14 is provided on an outer peripheral surface thereof with aplurality of rotational guide projections 14 c, and the helicoid ring 18is provided on an inner peripheral surface thereof with acircumferential groove 18 d (see FIGS. 4 and 8) in which the pluralityof rotational guide projections 14 c are engaged. Additionally, theannular flange 14 a is in contact with the rear end surface of thehelicoid ring 18 to be slidable thereon. Accordingly, due to theengagement of the plurality of rotational guide projections 14 c withthe circumferential groove 18 d and the engagement of the annular flange14 a with the rear end surface of the helicoid ring 18, the first linearguide ring 14 and the helicoid ring 18 are coupled to each other to beintegrally movable in the optical axis direction while allowing rotationof the helicoid ring 18 relative to the first linear guide ring 14.

The first linear guide ring 14 is provided with a set of three linearguide slots (through-slots) 14 d which extend parallel to the imagingoptical axis Z1. The zoom lens 5 is provided inside the helicoid ring 18with a third lens group moving ring 15. The third lens group moving ring15 is provided at the rear end thereof with an annular flange 15 a whichprojects radially outwards. The third lens group moving ring 15 isguided linearly in the optical axis direction due to the engagement of aset of three linear guide keys 15 b which project radially outward fromthe annular flange 15 a of the third lens group moving ring 15 with theset of three linear guide slots 14 d of the first linear guide ring 14.Each linear guide slot 14 d is formed through the first linear guidering 14 in a radial direction, and the third lens group moving ring 15is provided with a plurality of third-lens-group-control cam followers15 c which are respectively formed on the linear guide keys 15 b andproject radially outwards from the set of three linear guide slots 14 d,respectively. The plurality of third-lens-group-control cam followers 15c are engaged in a corresponding plurality of third-lens-group guide camgrooves 18 e formed on an inner peripheral surface of the helicoid ring18, respectively. The plurality of third-lens-group-control camfollowers 15 c and the plurality of third-lens-group guide cam grooves18 e are used for moving the third lens group LG3 in the optical axisdirection. As shown in FIG. 8 that shows the shape of eachthird-lens-group guide cam groove 18 e in developed plan view, eachthird-lens-group guide cam groove 18 e is provided with amovement-control groove portion 18 e 1 and a circumferential grooveportion 18 e 2. The movement-control groove portion 18 e 1 of eachthird-lens-group guide cam groove 18 e is inclined to a plane orthogonalto the imaging optical axis Z1 at a predetermined angle of inclination,which includes a component (axial-direction component) in the axialdirection of the helicoid ring 18. The circumferential groove portion 18e 2 of each third-lens-group guide cam groove 18 e is communicativelyconnected to the rear end of the movement-control groove portion 18 e 1of the associated third-lens-group guide cam groove 18 e, lies in aplane orthogonal to the imaging optical axis Z1, and does not include acomponent in the axial direction of the helicoid ring 18. When theplurality of third-lens-group-control cam followers 15 c of the thirdlens group moving ring 15 are positioned in the movement-control grooveportions 18 e 1 of the plurality of third-lens-group guide cam grooves18 e, respectively, a rotation of the helicoid ring 18 causes the thirdlens group moving ring 15 that is guided linearly to move in the opticalaxis direction relative to the helicoid ring 18 and the first linearguide ring 14 in accordance with the contours of the movement-controlgroove portions 18 e 1 of the plurality of third-lens-group guide camgrooves 18 e. On the other hand, when the plurality ofthird-lens-group-control cam followers 15 c of the third lens groupmoving ring 15 are positioned in the circumferential groove portions 18e 2 of the plurality of third-lens-group guide cam grooves 18 e,respectively, the third lens group moving ring 15 does not move in theoptical axis direction even if the helicoid ring 18 rotates. Theplurality of third-lens-group guide cam grooves 18 e are provided asthree pairs of cam grooves 18 e at three different positions in thecircumferential direction of the helicoid ring 18, and each pair of camgrooves 18 e includes a front cam groove 18 e and a rear cam groove 18 epositioned behind the front cam groove 18 e in the optical axisdirection. In each pair of cam grooves 18 e, the circumferential grooveportion 18 e 2 of the rear cam groove 18 e is open at the rear end ofthe helicoid ring 18. To correspond to the plurality of third-lens-groupguide cam grooves 18 e, the plurality of third-lens-group-control camfollowers 15 c consist of three pairs of cam followers 15 c at threedifferent positions in the circumferential direction of the third lensgroup moving ring 15, and each pair of cam followers 15 c consists of afront cam follower 15 c and a rear cam follower 15 c positioned behindthe front cam follower 15 c in the optical axis direction.

The zoom lens 5 is provided inside the third lens group moving ring 15with a third lens group frame 16 which is pivoted about a pivot shaft 17which extends parallel to the imaging optical axis Z1. The third lensgroup frame 16 holds the third lens group LG3 at a position eccentric tothe pivot shaft 17 and the optical axis of the third lens group LG3extends parallel to the pivot shaft 17. The third lens group frame 16 isrotatable (swingable) about the pivot shaft 17 between an on-axisposition (photographing position) shown in FIGS. 2, 3, 17, 19 and 20 atwhich the optical axis (center) of the third lens group LG3 coincideswith the imaging optical axis Z1, and an off-axis displaced position(radially retracted away from the imaging optical axis Z1) shown inFIGS. 1, 15, 18 and 22 at which the optical axis of the third lens groupLG3 is eccentrically positioned with respect to the imaging optical axisZ1. The third lens group frame 16 is biased to rotate in a directiontoward the on-axis position by a torsion spring 39 and held in theon-axis position by a stopper mechanism which will be discussed later.On the other hand, a rearward movement of the third lens group movingring 15 in the optical axis direction causes the third lens group frame16 to come into contact with a position-control cam bar 21 a whichprojects forward from the image sensor holding unit 21, and a furtherrearward movement of the third lens group moving ring 15 in the opticalaxis direction causes the third lens group frame 16 to rotate to theoff-axis displaced position against the biasing force of the torsionspring 39. More specifically, as shown in FIG. 14, a retracting camsurface 21 b having a predetermined degree of inclination relative tothe optical axis direction is formed on a front end surface of theposition-control cam bar 21 a, and a cam surface 16 a which faces theretracting cam surface 21 b when the third lens group frame 16 is in theon-axis position is formed on the third lens group frame 16. Upon thethird lens group moving ring 15 coming near to the image sensor holdingunit 21 while moving rearward, the retracting cam surfaces 21 b and 16 acome into contact with each other so that a component force whichrotates the third lens group frame 16 is produced from the rearwardmoving force in the optical axis direction, thus displacing the thirdlens group frame 16 to the off-axis displaced position. As shown in FIG.15, the third lens group frame 16 (the third lens group LG3) having beenrotated to the off-axis displaced position is accommodated in a lowerposition where the third lens group frame 16 does not interfere witheither the fourth lens group LG4 or the image sensor 60. In addition,the zoom lens 5 is provided inside the third lens group moving ring 15with a shutter unit 20 which is fixed to the third lens group movingring 15 to be positioned in front of the third lens group frame 16.Although not shown in the cross sectional views in FIGS. 1 through 3, ashutter and an adjustable diaphragm are incorporated in the shutter unit20.

As shown in FIG. 9, the first linear guide ring 14 is provided with aset of three roller-guiding cam slots 14 e which are formed throughinner and outer peripheral surfaces of the first linear guide ring 14.The zoom lens 5 is provided radially inside the first linear guide ring14 with a cam ring (first rotational member) 11 (a developed shape ofwhich is shown in FIG. 10) rotatable about the optical axis Z1. A set ofthree guide rollers (followers) 26 fixed to the cam ring 11 at differentcircumferential positions thereon to project radially outwards areslidably engaged in the set of three roller-guiding cam slots 14 e,respectively. The set of three guide rollers 26 extend radially outwardsthrough the set of three roller-guiding cam slots 14 e to be engaged ina set of three roller-engaging grooves 18 f which are formed on an innerperipheral surface of the helicoid ring 18, respectively. Although theset of three roller-engaging grooves 18 f that are formed on the innerperipheral surface of the helicoid ring 18 are located on the innersurface that should not visible in FIG. 8 (i.e., should be shown withbroken lines), that shows a developed plan view of the outer peripheralsurface of the helicoid ring 18, the set of three roller-engaginggrooves 18 f are shown by solid lines for the sake of clarity. As can beunderstood from FIG. 8, each roller-engaging groove 18 f is providedwith a rotational transfer groove portion 18 f 1 and a circumferentialgroove portion 18 f 2. The rotational transfer groove portion 18 f 1extends parallel to the imaging optical axis Z1 (i.e., includes anaxial-direction component), while the circumferential groove portion 18f 2 is communicatively connected to the rear end of the rotationaltransfer groove portion 18 f 1 of the associated roller-engaging groove18 f, lies in a plane orthogonal to the imaging optical axis Z1 and doesnot have axial-direction component. When the set of three guide rollers26 are engaged in the rotational transfer groove portions 18 f 1 of theset of three roller-engaging grooves 18 f, respectively, a rotationalforce of the helicoid ring 18 is transferred to the set of three guiderollers 26 via wall surfaces of the rotational transfer groove portion18 f 1 of each roller-engaging groove 18 f when the helicoid ring 18 isrotated, and accordingly, the cam ring 11 rotates integrally with thehelicoid ring 18 when the helicoid ring 18 is rotated. This rotation ofthe cam ring 11 causes the cam ring 11 to move in the optical axisdirection while rotating relative to the helicoid ring 18 and the firstlinear guide ring 14 in accordance with the contours of the set of threeroller-guiding cam slots 14 e, in which the set of three guide rollers26 are engaged, respectively. On the other hand, when the set of threeguide rollers 26 are engaged in the circumferential groove portions 18 f2 of the set of three roller-engaging grooves 18 f, respectively, norotational force of the helicoid ring 18 is transferred to the cam ring11 even if the helicoid ring 18 rotates, because each guide roller 26moves in the associated circumferential groove portion 18 f 2.

Rotating the zoom gear 28 by the zoom motor 150 in a lens barreladvancing direction causes the helicoid ring 18 to advance whilerotating due to the engagement of the inner helicoid 22 a with the outerhelicoid 18 a. This advancing and rotating movement of the helicoid ring18 causes the first linear guide ring 14 to move linearly forward withthe helicoid ring 18. Upon the helicoid ring 18 and the first linearguide ring 14 being advanced by a predetermined amount of movement, theouter helicoid 18 a and the inner helicoid 22 a are disengaged from eachother, and the set of three guide projections 18 b of the helicoid ring18 are engaged in the circumferential groove 22 b of the stationarybarrel 22, so that the helicoid ring 18 does not move in the opticalaxis direction relative to the stationary barrel 22 and only rotates ata fixed position in the optical axis direction. Consequently, the firstlinear guide ring 14, which has advanced in the optical axis direction,is also stopped at a fixed position.

The position of the third lens group moving ring 15 in the optical axisdirection, which is guided linearly by the first linear guide ring 14,is controlled by the plurality of third-lens-group guide cam grooves 18e of the helicoid ring 18. Namely, when the plurality ofthird-lens-group-control cam followers 15 c are positioned in thecircumferential groove portions 18 e 2 of the plurality ofthird-lens-group guide cam grooves 18 e, respectively, the relativeposition between the helicoid ring 18 and the third lens group movingring 15 in the optical axis direction does not vary; however, theabsolute position of the third lens group moving ring 15 in the opticalaxis direction relative to the image sensor holding unit 21 varies inaccordance with variations in position of the helicoid ring 18 that isadvanced due to the engagement between the outer helicoid 18 a and theinner helicoid 22 a. In addition, once the plurality ofthird-lens-group-control cam followers 15 c enter the movement-controlgroove portions 18 e 1 of the plurality of third-lens-group guide camgrooves 18 e, respectively, a rotation of the helicoid ring 18 causesthe third lens group moving ring 15 to move relative to the helicoidring 18 in the optical axis direction in accordance with the contours ofthe movement-control groove portions 18 e 1.

The position of the cam ring 11 in the optical axis direction iscontrolled by the engagement of the set of three roller-guiding camslots 14 e of the first linear guide ring 14 with the set of threeroller-engaging grooves 18 f that are formed on an inner peripheralsurface of the helicoid ring 18. Namely, when the set of three guiderollers 26 are engaged in the circumferential groove portions 18 f 2 ofthe set of three roller-engaging grooves 18 f, respectively, the camring 11 does not rotate with the helicoid ring 18, so that the positionof the cam ring 11 relative to the helicoid ring 18 in the optical axisdirection does not change. However, the absolute position of the camring 11 relative to the image sensor holding unit 21 in the optical axisdirection varies in accordance with variations in position of thehelicoid ring 18 that is advanced due to the engagement between theouter helicoid 18 a and the inner helicoid 22 a. Upon the set of threeguide rollers 26 entering the rotational transfer groove portions 18 f 1of the set of three roller-engaging grooves 18 f, respectively, arotation of the helicoid ring 18 causes the cam ring 11 to rotate withthe helicoid ring 18, thus causing the cam ring 11 to move in theoptical axis direction while rotating relative to the first linear guidering 14 in accordance with the contours of the set of threeroller-guiding cam slots 14 e.

As shown in FIG. 4, the linear guide ring 14 is provided on an innerperipheral surface thereof with a plurality of linear guide grooves(bottomed grooves) 14 f which extend parallel to the imaging opticalaxis Z1, independently of the set of three linear guide slots 14 d thatare used for guiding the third lens group moving ring 15 linearly in theoptical axis direction. The zoom lens 5 is provided inside the firstlinear guide ring 14 with a second linear guide ring 10 and a middleexternal barrel (middle advancing barrel) 13, each of which is guidedlinearly in the optical axis direction by the plurality of linear guidegrooves 14 f. The middle external barrel 13 advances from and retractsinto the helicoid ring 18.

The second linear guide ring 10 is provided with a rear end flange 10 a,a small-diameter flange 10 b and a pair of guide keys 10 c. The rear endflange 10 a is formed in a plane substantially orthogonal to the imagingoptical axis Z1, the small-diameter flange 10 b is positioned in frontof the rear end flange 10 a with a predetermined spacing therebetween,and the pair of guide keys 10 c project forward from the position of thesmall-diameter flange 10 a, to extend parallel to the imaging opticalaxis Z1. The rear end flange 10 a is provided with a plurality of linearguide projections 10 d which project radially outwards. The secondlinear guide ring 10 can be guided linearly in the optical axisdirection in a state in which the plurality of linear guide projections10 d being engaged in the plurality of linear guide grooves 14 f to beslidingly movable therein in the optical axis direction. The cam ring 11is provided at the rear end thereof with a plurality of rotation guideprojections 11 a (see FIGS. 17 through 22) which project radiallyinwards and are engaged in between the rear end flange 10 a and thesmall-diameter flange portion 10 b in a manner to be prevented frommoving in the optical axis direction relative to the second linear guidering 10 and to be allowed to rotate relative to the second linear guidering 10. Due to this engagement, the cam ring 11 and the second linearguide ring 10 are coupled to each other to be rotatable relative to eachother and to be movable together in the optical axis direction. The zoomlens 5 is provided inside the cam ring 11 with a second lens groupholding ring 8 which holds the second lens group LG2. The pair of guidekeys 10 c of the second linear guide ring 10 are engaged in a pair oflinear grooves 8 a, respectively, which are formed on the second lensgroup holding ring 8 to extend parallel to the imaging optical axis Z1.Due to the engagement of the pair of guide keys 10 c with the pair oflinear grooves 8 a, the second lens group holding ring 8 is guidedlinearly in the optical axis direction. The second lens group holdingring 8 is provided on an outer peripheral surface thereof with a set ofthree second-lens-group-control cam followers 8 b (only two of whichappear in FIG. 5) which are respectively engaged in a set of threesecond-lens-group guide cam grooves 11 b formed on an inner peripheralsurface of the cam ring 11. A rotation of the cam ring 11 causes thesecond lens group moving ring 8 to move in the optical axis directiondue to the engagement of the set of three second-lens-group guide camgrooves 11 b with the set of three second-lens-group-control camfollowers 8 b.

The middle external barrel 13 is provided, on an outer peripheralsurface thereof in the vicinity of the rear end of the middle externalbarrel 13, with an annular flange 13 a, and is provided on the annularflange 13 a with a plurality of linear guide projections 13 b which areslidably engaged in the plurality of linear guide grooves 14 f of thefirst linear guide ring 14, respectively. The middle external barrel 13is guided linearly in the optical axis direction due to the engagementof the plurality of linear guide projections 13 b with the plurality oflinear guide grooves 14 f. The middle external barrel 13 is provided onan inner peripheral surface thereof with a set of three linear guidegrooves 13 c which extend parallel to the imaging optical axis Z1, andis provided at the rear end of the inner peripheral surface of themiddle external barrel 13 with a plurality of rotation guide projections13 d which project radially inwards. The cam ring 11 is provided, on anouter peripheral surface thereof in the vicinity of the rear end of thecam ring 11, with a rear end flange 11 c which projects radiallyoutwards and a small-diameter flange 11 d positioned in front of therear end flange 11 c with a predetermined spacing provided therebetween.The radially inner ends (fixed ends) of the set of three guide rollers26 are embedded into the rear end flange 11 c. The plurality of rotationguide projections 13 d are engaged in between the rear end flange 11 cand the small-diameter flange 11 d in a manner to be prevented frommoving in the optical axis direction relative to the cam ring 11 and tobe allowed to rotate relative to the cam ring 11. Due to thisengagement, the cam ring 11 and the middle external barrel 13 arecoupled to each other to be rotatable relative to each other and to bemovable together in the optical axis direction. The zoom lens 5 isprovided immediately inside the middle external barrel 13 with afrontmost external barrel (inner advancing barrel) 12 which advancesfrom and retracts into the middle external barrel 13. The frontmostexternal barrel 12, a developed shape of which is shown in FIG. 11, isprovided on an outer peripheral surface thereof in the vicinity of therear end of the frontmost external barrel 12 with a set of three linearguide projections 12 a which are engaged in the set of three linearguide grooves 13 c of the middle external barrel 13. The frontmostexternal barrel 12 is guided linearly in the optical axis direction dueto the engagement of the set of three linear guide projections 12 a withthe set of three linear guide grooves 13 c.

The frontmost external barrel 12 is provided on an inner peripheralsurface thereof with a set of three linear guide grooves 12 b (see FIG.11) which are formed to extend parallel to the imaging optical axis Z1.The zoom lens 5 is provided inside the frontmost external barrel 12 witha first lens group holding ring 19 which holds the first lens group LG1.The first lens group holding ring 19, a developed shape of which isshown in FIG. 12, is provided on an outer peripheral surface thereofwith a set of three guide projections 19 a which are engaged in the setof three linear guide grooves 12 b to be movable therein in the opticalaxis direction, respectively. Due to the engagement of the set of threelinear guide grooves 12 b with the set of three guide projections 19 a,the frontmost external barrel 12 and the first lens group holding ring19 are coupled to each other in a manner to be prevented from rotatingrelative to each other and to be allowed to move relative to each otherin the optical axis direction. Namely, the first lens group holding ring19 is also guided linearly in the optical axis direction via thefrontmost external barrel 12. The first lens group holding ring 19 isprovided on an outer peripheral surface thereof with a set of threelinear grooves 19 b which extend rearward from the set of three guideprojections 19 a, respectively. The set of three linear grooves 19 b andthe set of three linear guide grooves 12 b form three springaccommodation spaces 27 in which three first lens group biasing springs23 are accommodated, respectively. Each of the three first lens groupbiasing springs 23 is a compression coil spring. The front and rear endsof each first lens group biasing spring 23 are in contact with theassociated guide projection 19 a and the rear end surface of theassociated linear guide groove 12 b, respectively.

The frontmost external barrel 12 is provided, on an inner peripheralsurface thereof in the vicinity of the rear end of the frontmostexternal barrel 12, with a set of three lead projections 12 c, and thefirst lens group holding ring 19 is provided on an inner peripheralsurface thereof with three pairs of first-lens-group-control camfollowers 19 c (see FIG. 12). The set of three lead projections 12 c areslidably engaged in a set of three first lead cam grooves 11 e,respectively, which are formed on an outer peripheral surface of the camring 11, while the three pairs of first-lens-group-control cam followers19 c are slidably engaged in three pairs of second lead cam grooves 11f, respectively, which are formed on an outer peripheral surface of thecam ring 11. A rotation of the cam ring 11 causes the set of three leadprojections 12 c to move along the set of three first lead cam grooves11 e therein while being guided thereby, respectively, thus causing thefrontmost external barrel 12 to move in the optical axis directionrelative to the cam ring 11. In addition, a rotation of the cam ring 11causes the three pairs of first-lens-group-control cam followers 19 c tomove along the three pairs of second lead cam grooves 11 f therein whilebeing guided thereby, respectively, thus causing the first lens groupholding ring 19 to move in the optical axis direction relative to thecam ring 11.

An advancing operation and a retracting operation of the zoom lens 5that has the above described structure will be discussed hereinafter.Rotating the zoom gear 28 in the lens barrel advancing direction via thezoom motor 150 from the retracted state (fully-retracted state) of thezoom lens 5 shown in FIG. 1 causes the helicoid ring 18 to move forwardwhile rotating relative to the stationary barrel 22 due to theengagement between the outer helicoid 18 a of the helicoid ring 18 andthe inner helicoid 22 a of the stationary barrel 22. The first linearguide ring 14 moves linearly forward with the helicoid ring 18. Uponbeing advanced to a predetermined forward position thereof, the helicoidring 18 and the first linear guide ring 14 both stop moving in theoptical axis direction, and thereafter the helicoid ring 18 rotates atan axially fixed position due to the engagement of the set of threeguide projections 18 b with the circumferential groove 22 b.

When the zoom lens 5 is in the retracted state, the plurality ofthird-lens-group-control cam followers 15 c of the third lens groupmoving ring 15 are positioned in the circumferential groove portions 18e 2 of the plurality of third-lens-group guide cam grooves 18 e,respectively, and accordingly, each third-lens-group-control camfollower 15 c relatively moves in the circumferential groove portion 18e 2 of the associated third-lens-group guide cam groove 18 e for a whilefrom the moment the helicoid ring 18 starts advancing while rotatingfrom the retracted position thereof. Therefore, the relative positionbetween the helicoid ring 18 and the third lens group moving ring 15 inthe optical axis direction does not change, while the third lens groupmoving ring 15 is moved forward in the optical axis direction by anamount of forward movement of the helicoid ring 18 caused by theengagement between the outer helicoid 18 a of the helicoid ring 18 andthe inner helicoid 22 a of the stationary barrel 22. Subsequently, uponthe helicoid ring 18 being rotated at a predetermined angle of rotationin a lens barrel advancing direction, the plurality ofthird-lens-group-control cam followers 15 c enter into themovement-control groove portions 18 e 1 of the plurality ofthird-lens-group guide cam grooves 18 e, respectively. Thereupon, thethird lens group moving ring 15 moves in the optical axis directionrelative to the helicoid ring 18 in a predetermined moving manner whilebeing guided by the movement-control groove portions 18 e 1 inaccordance with the rotation of the helicoid ring 18. In the zoomingrange from the wide-angle extremity shown in FIG. 2 to the telephotoextremity shown in FIG. 3, the plurality of third-lens-group-control camfollowers 15 c are guided by the movement-control groove portions 18 e 1of the plurality of third-lens-group guide cam grooves 18 e,respectively.

In the state shown in FIG. 1, in which the zoom lens 5 is in theretracted state, the third lens group frame 16, which is positionedinside the third lens group moving ring 15, is held at the off-axisdisplaced position, in which the optical axis of the third lens groupLG3 is eccentricity positioned downward with respect to the imagingoptical axis Z1, via the position-control cam bar 21 a that is formed toproject forward from the image sensor holding unit 21 (see FIG. 15).During the course of movement of the third lens group moving ring 15from the retracted position to the wide-angle extremity position in thezooming range, the third lens group frame 16 is disengaged from theposition-control cam bar 21 a of the image sensor holding unit 21 torotate about the pivot shaft 17 from the off-axis displaced position tothe on-axis position, in which the optical axis of the third lens groupLG3 coincides with the imaging optical axis Z1, via the spring force ofthe torsion spring 39. Thereafter, the third lens group frame 16 remainsheld at the on-axis position until the zoom lens 5 is again retracted tothe retracted position (the position shown in FIG. 1).

Additionally, when the zoom lens 5 is in the retracted state, the set ofthree guide rollers 26 are positioned in the circumferential grooveportions 18 f 2 of the set of three roller-engaging grooves 18 f,respectively, and accordingly, each guide roller 26 relatively moves inthe associated circumferential groove portion 18 f 2 for a while fromthe moment the helicoid ring 18 starts advancing while rotating from theretracted position thereof. Therefore, the relative position between thehelicoid ring 18 and the cam ring 11 in the optical axis direction doesnot change, while the cam ring 11 is moved forward in the optical axisdirection by an amount of forward movement of the helicoid ring 18caused by the engagement between the outer helicoid 18 a of the helicoidring 18 and the inner helicoid 22 a of the stationary barrel 22. At thisstage, the cam ring 11 is moved linearly without changing the angularposition thereof because the set of three guide rollers 26 are engagedwith the rear ends of the set of three roller-guiding cam slots 14 e tothereby be prevented from rotating. Subsequently, upon the helicoid ring18 being rotated at a predetermined angle of rotation in a lens barreladvancing direction, the set of three guide rollers 26 enter into therotational transfer groove portions 18 f 1 of the set of threeroller-engaging grooves 18 f, respectively. Thereupon, the cam ring 11starts rotating with the helicoid ring 18, and the cam ring 11 is movedin the optical axis direction relative to the helicoid ring 18 and thefirst linear guide ring 14 in a predetermined moving manner while beingguided by the set of three roller-guiding cam slots 14 e of the firstlinear guide ring 14. In the zooming range from the wide-angle extremityshown in FIG. 2 to the telephoto extremity shown in FIG. 3, the set ofthree guide rollers 26 are positioned in the rotational transfer grooveportions 18 f 1 of the set of three roller-engaging grooves 18 f,respectively, so that the cam ring 11 is rotated in association with thehelicoid ring 18 whenever the helicoid ring 18 rotates.

Namely, in the advancing operation of the zoom lens 5 from the retractedstate, the helicoid ring 18 starts form a advancing rotational state inwhich the helicoid ring 18 moves forward in the optical axis directionwhile rotating about the imaging optical axis Z1. Subsequently, thehelicoid ring 18 changes to a fixed-position rotational state in whichthe helicoid ring 18 rotates without changing the position thereof inthe optical axis direction. The third lens group moving ring 15 startsfrom a first linear moving state in which the third lens group movingring 15 moves forward linearly in the optical axis direction with thehelicoid ring 18, and subsequently, the third lens group moving ring 15changes to a second linear moving state in which the third lens groupmoving ring 15 changes the position thereof relative to the helicoidring 18 in the optical axis direction (in which the movement of thethird lens group moving ring 15 is controlled by the movement-controlgroove portions 18 e 1 of the plurality of third-lens-group guide camgrooves 18 e). The cam ring 11 starts from a linear moving state inwhich the cam ring 11 moves forward linearly in the optical axisdirection with the helicoid ring 18, and subsequently, the cam ring 11changes to a advancing/retracting rotational state in which the cam ring11 moves in the optical axis direction relative to the helicoid ring 18while rotating with the helicoid ring 18 (in which the movement of thecam ring 11 is controlled by the set of three roller-guiding cam slots14 e of the first linear guide ring 14). In the ready-to-photographstate of the zoom lens 5 (in the zooming range from the wide-angleextremity shown in FIG. 2 to the telephoto extremity shown in FIG. 3),the helicoid ring 18 is in the fixed-position rotational state, thethird lens group moving ring 15 is in the second linear moving state,and the cam ring 11 is in the advancing/retracting rotational state.

A rotation of the cam ring 11 causes the second lens group holding ring8, which is guided linearly in the optical axis direction via the secondlinear guide ring 10, to move in the optical axis direction in apredetermined moving manner in the inner side of the cam ring 11 due tothe engagement of the set of three second-lens-group guide cam grooves11 b with the set of three second-lens-group-control cam followers 8 b.In addition, this rotation of the cam ring 11 causes the frontmostexternal barrel 12, that is guided linearly in the optical axisdirection via the middle external barrel 13, to move in the optical axisdirection in a predetermined moving manner due to the engagement of theset of three lead projections 12 c with the set of three first lead camgrooves 11 e, and further causes the first lens group holding frame 19,which is guided linearly in the optical axis direction via the frontmostexternal barrel 12, to move in the optical axis direction in apredetermined moving manner due to the engagement of the three pairs offirst-lens-group-control cam followers 19 c with the three pairs ofsecond lead cam grooves 11 f.

Due to the above-described configuration, an axial position of the firstlens group LG1 relative to an imaging surface (light-receiving surface)of the image sensor 60 when the first lens group LG1 is moved forwardfrom the retracted position is determined by the sum of the amount offorward movement of the cam ring 11 relative to the stationary barrel 22and the amount of movement (caused by cam) of the first lens groupholding ring 19 relative to the cam ring 11, while an axial position ofthe second lens group LG2 relative to the imaging surface of the imagesensor 60 when the second lens group LG2 is moved forward from theretracted position is determined by the sum of the amount of forwardmovement of the cam ring 11 relative to the stationary barrel 22 and theamount of movement (caused by cam) of the second lens group holding ring8 relative to the cam ring 11. An axial position of the third lens groupLG3 relative to the imaging surface of the image sensor 60 when thethird lens group LG3 is moved forward from the retracted position isdetermined by the sum of the amount of forward movement of the helicoidring 18 relative to the stationary barrel 22 and the amount of movement(caused by cam) of the third lens group moving ring 15 relative to thehelicoid ring 18. A zooming operation is carried out by moving thefirst, second and third lens groups LG1, LG2 and LG3 along the imagingoptical axis Z1 while changing the air-distances therebetween. When thezoom lens 5 is driven to advance from the fully-retracted position shownin FIG. 1, the zoom lens 5 firstly extends into the state shown in FIG.2, in which the zoom lens 5 is set at the wide-angle extremity.Subsequently, the zoom lens 5 moves into the state shown in FIG. 3, inwhich the zoom lens 5 is set at the telephoto extremity as shown in FIG.3 by a further rotation of the zoom motor 150 in the lens barreladvancing direction.

When the first through fourth lens groups LG1, LG2, LG3 and LG4 arepositioned in the zooming range (i.e., in the ready-to-photographstate), a focusing operation is carried out by moving the AF lens frame51, which holds the fourth lens group LG4, along the imaging opticalaxis Z1 by rotation of the AF motor 160 in accordance with an objectdistance.

Driving the zoom motor 150 in the lens barrel retracting directioncauses the zoom lens 5 to operate in the reverse manner to the abovedescribed advancing operation, i.e., to perform a retracting operationso that each annular movable member of the zoom lens 5 is moved rearwardin the optical axis direction. During the course of this retractingmovement of the zoom lens 5, the third lens group frame 16 rotates aboutthe pivot shaft 17 to the off-axis displaced position via theposition-control cam bar 21 a while moving rearward with the third lensgroup moving ring 15. When the third lens group moving ring 15 isretracted to the retracted position shown in FIG. 1, the third lensgroup LG3 is retracted into space radially outside the space in whichthe fourth lens group LG4, the low-pass filter LF and the image sensor60 are retracted as shown in FIG. 1 (namely, the third lens group LG3 isradially retracted into an axial range substantially identical to anaxial range in the optical axis direction in which the fourth lens groupLG4, the low-pass filter LF and the CCD image sensor 60 are positioned).This structure of the zoom lens 5 for retracting (displacing) the thirdlens group LG3 in this manner reduces the length of the zoom lens 5 whenthe zoom lens 5 is fully retracted.

The zoom lens 5 is provided at the front end thereof with a lens barriermechanism for shutting the front of the first lens group LG1 when in theretracted state, in which no pictures are taken. The lens barriermechanism is provided with a barrier mount frame 41, a pair of innerbarrier blades 45, a pair of outer barrier blades 46, a pair of torsionsprings 47, a barrier retaining plate 42 and a barrier drive ring 43.The barrier mount frame 41 is fixed at the front end of the frontmostexternal barrel 12 and includes a photographing aperture 41 a. The pairof inner barrier blades 45 and the pair of outer barrier blades 46 aresupported by the barrier mount frame 41 thereon in a manner to becapable of opening and shutting the photographing aperture 41 a. Thepair of torsion springs 47 bias the pair of inner barrier blades 45 andthe pair of outer barrier blades 46 in directions to shut thephotographing aperture 41 a. The barrier retaining plate 42 holds thepair of inner barrier blades 45, the pair of outer barrier blades 46 andthe pair of torsion springs 47 with these elements being positionedbetween the barrier mount frame 41 and the barrier retaining plate 42.The barrier drive ring 43 is positioned immediately behind the barrierretaining plate 42 and is supported by the barrier retaining plate 42 ina manner to be capable of rotating about the imaging optical axis Z1relative to the barrier retaining plate 42. More specifically, the pairof inner barrier blades 45 and the pair of outer barrier blades 46 arepivoted about a pair of pivot pins 41 b (see FIGS. 29 and 30) providedon the barrier mount frame 41 to be freely swingable about the pair ofpivot pins 41 b, respectively, and the pair of inner barrier blades 45are biased to rotate in directions to shut the photographing aperture 41a by the pair of torsion springs 47. Each of the pair of outer barrierblades 46 is provided on the radially outer edge thereof with a linkageprojection 49, and each inner barrier blade 45 and the associated outerbarrier blade 46 are linked with each other via the linkage projection49 thereof in either of the shutter opening and shutting directions.

The barrier drive ring 43 is provided in the vicinity of the pair ofpivot pins 41 b with a pair of barrier drive pins 44, respectively,which project forward (see FIG. 6). When the barrier drive ring 43 isrotated in a direction shown by the arrow F1 in FIG. 29, the pair ofbarrier drive pins 44 come into contact with the pair of inner barrierblades 45, respectively, so that each barrier drive pin 44 applies aforce in the barrier opening direction to the associated inner barrierblade 45. The barrier drive ring 43 is biased to rotate in the barrieropening direction (i.e., in the direction of the arrow F1 shown in FIG.29) by three extension coil springs 48 which are stretched and installedbetween three spring hooks 43 a of the barrier drive ring 43 and threespring hooks 19 e of the first lens group holding ring 19, respectively.The spring force of the three extension coil springs 48 is greater thanthe spring force of the pair of torsion springs 47. When the barrierdrive ring 43 is positioned at the limit of rotation thereof (limit ofclockwise rotation with respect to FIG. 29) by the biasing force of thethree extension coil springs 48, the pair of barrier drive pins 44 pressand open the pair of inner barrier blades 45 against the biasing forceof the pair of torsion springs 47, thus also opening the pair of outerbarrier blades 46 via the linkage projections 49 thereof, respectively(see FIG. 30).

The barrier drive ring 43 is provided with a rotational transferprojection 43 b which projects rearward as shown in FIG. 28. Therotational transfer projection 43 b is engageable with and disengagedfrom a rotation-transmitting stepped portion 50 preselected from amongthree rotation-transmitting stepped portions 50 formed at the front endof the cam ring 11. Since the barrier drive ring 43 is supported to berotatable at a fixed position in the optical axis direction relative tothe frontmost external barrel 12, the relative position between therotational transfer projection 43 b and the specific (preselected)rotation-transmitting stepped portion 50 in the optical axis directionvaries when the frontmost external barrel 12 moves linearly in theoptical axis direction in accordance with rotation of the cam ring 11due to the engagement of the set of three lead projections 12 c with theset of three first lead cam grooves 11 e. The rotational transferprojection 43 b is not engaged with the specific (preselected)rotation-transmitting stepped portion 50 in the zooming range from thewide-angle extremity to the telephoto extremity. The rotational transferprojection 43 b and the specific (preselected) rotation-transmittingstepped portion 50 come into engagement with each other at a pointduring the retracting operation of the zoom lens 5 from the wide-angleextremity to the retracted position. Thereupon, a forced-rotation forcein a direction opposite to the direction of the biasing force of thethree extension coil springs 48 is transmitted to the rotationaltransfer projection 43 b from the specific (preselected)rotation-transmitting stepped portion 50. When the barrier drive ring 43is rotated to the other limit of rotation (limit of counterclockwiserotation with respect to FIG. 29) against the biasing force of the threeextension coil springs 48, the pair of barrier drive pins 44 aredisengaged from the pair of inner barrier blades 45 to shut the pair ofinner barrier blades 45 by the biasing force of the pair of torsionsprings 47, thus also shutting the pair of outer barrier blades 46 viathe linkage projections 49 thereof, respectively. Consequently, thephotographing aperture 41 a is shut (closed) (see FIGS. 1, 29 and 32).Conversely, when the zoom lens 5 moves into the zooming range from theretracted position, the rotational transfer projection 43 b isdisengaged from the specific (preselected) rotation-transmitting steppedportion 50 so that the barrier drive ring 43 rotates in the barrieropening direction by the biasing force of the three extension coilsprings 48, and consequently, the pair of barrier drive pins 44 pressand open the pair of inner barrier blades 45, thus also opening the pairof outer barrier blades 46 via the interlock projections 29 thereof,respectively. Accordingly, the pair of inner barrier blades 45 and thepair of outer barrier blades 46 are opened and shut by clockwise andcounterclockwise rotations of the barrier drive ring 43, and the barrierdrive ring 43 rotates via the cam ring 11 in a direction to shut thepair of inner barrier blades 45 and the pair of outer barrier blades 46.As shown in FIG. 10, the three rotation-transmitting stepped portions 50of the cam ring 11 are arranged at substantially equi-angular intervalsof 120 degrees, and one rotation-transmitting stepped portion 50 to beused for controlling the rotation of the barrier drive ring 43 can befreely selected from among the three rotation-transmitting steppedportions 50 upon the lens barrier mechanism being installed into thefront end of the zoom lens 5.

The zoom lens 5 is provided between the second lens group LG2 and thethird lens group LG3 with an inter-lens-group biasing spring 34. Thesecond lens group holding ring 8 is provided in the vicinity of thefront end thereof with a ring-shaped inner flange 8 c, and a springsupport ring 35 is fixed to the third lens group moving ring 15 so as toface the inner flange 8 c. The spring support ring 35 is fixed at thefront end of the third lens group moving ring 15 to support the shutterunit 20 onto the third lens group moving ring 15. The inter-lens-groupbiasing spring 34 is a truncated-conical compression coil spring thediameter of which reduces in the rearward direction from the front inthe optical axis direction. The inter-lens-group biasing spring 34 isinstalled between the inner flange 8 c and the spring support ring 35 ina compressed fashion to increase the distance between the second lensgroup LG2 and the third lens group LG3 with the front and rear ends ofthe inter-lens-group biasing spring 34 being in contact with the innerflange 8 c of the second lens group holding ring 8 and the springsupport ring 35, respectively.

FIG. 13 shows a biasing force transfer mechanism, using theinter-lens-group biasing spring, 34 when the zoom lens 5 is set at thewide-angle extremity in the ready-to-photograph state. The second lensgroup holding ring 8 is biased forward by the inter-lens-group biasingspring 34, so that the set of three second-lens-group-control camfollowers 8 b are pressed against the front side walls in the set ofthree second-lens-group guide cam grooves 11 b, respectively, Thiscauses the cam ring 11 to be pressed forward, thus causing the set ofthree guide rollers 26 that project from the cam ring 11 to be pressedagainst the front side walls in the set of three roller-guiding camslots 14 e, respectively, that are formed on the first linear guide ring14. Furthermore, the first linear guide ring 14 is pressed forward viathe set of three guide rollers 26, while the plurality of linear guideprojections 14 b that are formed in the vicinity of the rear end of thefirst linear guide ring 14 are pressed against the plurality of limitwall portions (rearward-facing limit surface) 22 e of the stationarybarrel 22, which are formed at the front ends of the plurality of linearguide groove 22 d, respectively. Namely, the zoom lens 5 is configuredso that the biasing force of the inter-lens-group biasing spring 34 istransferred from the second lens group holding ring 8 to the firstlinear guide ring 14 via the cam ring 11 (wherein the second lens groupholding ring 8, the cam ring 11 and the first linear guide ring 14 areelements of a cam mechanism (position control mechanism) for controllingthe position of the second lens group LG2 in the optical axisdirection), and so that backlash in the cam mechanism provided betweenthese annular members is eliminated while the biasing force of theinter-lens-group biasing spring 34 is ultimately received by theplurality of limit wall portions 22 e of the stationary barrel 22 thatis a stationary member of the zoom lens 5.

The third lens group moving ring 15 is biased rearward by theinter-lens-group biasing spring 34, so that the plurality ofthird-lens-group-control cam followers 15 c are pressed against the rearside walls in the plurality of third-lens-group guide cam grooves 18 eof the helicoid ring 18, respectively. This causes the helicoid ring 18to be pressed rearward, thus causing the set of three guide projections18 b that project from the helicoid ring 18 to be pressed against alimit wall portion (forward-facing limit surface) 22 f that correspondsto the rear side wall in the circumferential groove 22 b. Namely, thezoom lens 5 is configured so that the biasing force of theinter-lens-group biasing spring 34 is transferred from the third lensgroup moving ring 15 to the helicoid ring 18 (wherein the third lensgroup moving ring 15 and the helicoid ring 18 are elements of a cammechanism (position control mechanism) for controlling the position ofthe third lens group LG3 in the optical axis direction), and so thatbacklash in the cam mechanism provided between these annular members iseliminated while the biasing force of the inter-lens-group biasingspring 34 is ultimately received by the limit wall portion 22 f of thestationary barrel 22 that is a stationary member of the zoom lens 5.

FIG. 7 shows the positions “R1”, “W1” and “T1” of each guide projection18 b of the helicoid ring 18 when the zoom lens 5 is in the retractedstate, set at the wide-angle extremity and set at the telephotoextremity, respectively. In addition, FIG. 7 shows the positions “14b-R” and “14 b-Z” of each linear guide projection 14 b of the firstlinear guide ring 14 when the zoom lens 5 is in the retracted state andthe ready-to-photograph state in the zooming range (between thewide-angle extremity and the telephoto extremity), respectively. As canbe understood from FIG. 7, in the retracted state of the zoom lens 5,each linear guide projection 14 b is positioned in the vicinity of therear end of the associated linear guide groove 22 d while each guideprojection 18 b is positioned in the associated lead groove 22 c, sothat the linear guide projections 14 b are not in contact with theassociated limit wall portion 22 e of the stationary barrel 22 and theguide projections 18 b are not in contact with the associated limit wallportion 22 f of the stationary barrel 22. When the zoom lens 5 movesfrom the retracted state into the zooming range, each linear guideprojection 14 b is moved to the front end of the associated linear guidegroove 22 d, each guide projection 18 b enters the circumferentialgroove 22 b, and each linear guide projection 14 b and each guideprojection 18 b are pressed against the associated limit wall portion 22e and the limit wall portion 22 f by the biasing force of theinter-lens-group biasing spring 34, respectively. Accordingly, when thezoom lens 5 moves from the retracted state into the ready-to-photographstate, a backlash eliminating mechanism with each of the limit wallportions 22 e and 22 f that serves as a biasing force support comes intoaction. In the retracted state of the zoom lens 5, in which no picturesare taken, no problem arises even if the zoom lens 5 is structured sothat neither of the limit wall portions 22 e and 22 f receives thebiasing force of the inter-lens-group biasing spring 34 because accuracycontrol for the positions of the second lens group LG2 and the thirdlens group LG3 in the optical axis direction is not required to be soprecise as that in the ready-to-photograph state.

In this manner, simplification of the backlash eliminating mechanismwith less number of elements and with no need of a great number ofspring members is achieved since backlash between all the associatedmembers is eliminated with the single inter-lens-group biasing spring 34on each of the two cam mechanisms, i.e., the cam mechanism forcontrolling the position of the second lens group LG2 in the opticalaxis direction and the cam mechanism for controlling the position of thethird lens group LG3 in the optical axis direction.

On the stationary barrel 22, the limit wall portion 22 f, whichultimately receives a rearwardly pressing force applied to the thirdlens group LG3 (positioned behind the second lens group LG2), is formedat a forward position of the stationary barrel 22 with respect to eachlimit wall portion 22 e, which ultimately receives a forwardly pressingforce applied to the second lens group LG2 (positioned in front of thethird lens group LG3), in the optical axis direction. In other words,each limit wall portion 22 e provided at a relatively rearward positionreceives a forwardly pressing force of the inter-lens-group biasingspring 34, and the limit wall portion 22 f provided at a relativelyforward position receives a rearwardly pressing force of theinter-lens-group biasing spring 34, and hence, the limit wall portions22 e and 22 f are positioned on the stationary barrel 22 in a ‘reversed’manner with respect to the forward/rearward biasing directions of theinter-lens-group biasing spring 34 in the optical axis direction. Due tothis arrangement, the biasing force of the inter-lens-group biasingspring 34 acts on the second lens group holding ring 8 and the thirdlens group moving ring 15 in directions to cause the second lens groupholding ring 8 and the third lens group moving ring 15 to move away fromeach other while the biasing force of the inter-lens-group biasingspring 34 acts on the first linear guide ring 14 and the helicoid ring18 in directions to make the plurality of linear guide projections 14 band the set of three guide projections 18 b approach each other on thestationary barrel 22, and the direction of action of the biasing forceof the inter-lens-group biasing spring 34 is ultimately reversed andinput (applied) to the stationary barrel 22. By adopting this structure,compression loads act on a portion of the stationary barrel 22 betweeneach limit wall portion 22 e and the limit wall portion 22 f. Sincesynthetic resin which is widely used as a material for elements of alens barrel has a relatively high compression load strength, the zoomlens 5 is easily miniaturized by providing each limit wall portion 22 eand the limit wall portion 22 f close to each other by adopting theaforementioned structure in which compression loads act on a portion ofthe stationary barrel between each limit wall portion 22 e and the limitwall portion 22 f.

The accommodating structure for the third lens group LG3 will bediscussed hereinafter. As shown in FIG. 16, the third lens group movingring 15 is provided in front of the annular flange 15 a with acylindrical portion 15 d, and is provided inside the cylindrical portion15 d with a middle flange 15 e which extends radially inwards to besubstantially parallel to the annular flange 15 a. The third lens groupmoving ring 15 is further provided at the center of the middle flange 15e with a through hole 15 f which is formed through the middle flange 15e in the optical axis direction. The shutter unit 20 is fixed to thefront of the middle flange 15 e. The middle flange 15 e is provided onthe back thereof with a rotation limit pin 15 g which projects rearward.The third lens group moving ring 15 is provided, on the radiallyopposite side of the axis of the third lens group moving ring 15 fromthe rotation limit pin 15 g, with an accommodation space 15 h. Theaccommodation space 15 h includes a through-hole portion 15 h 1 and asemi-circular recessed portion 15 h 2. The through-hole portion 15 h 1is communicatively connected with the through hole 15 f and extendsthrough the cylindrical portion 15 d in a radial direction of the thirdlens group moving ring 15. The semi-circular recessed portion 15 h 2 iscommunicatively connected with the through-hole portion 15 h 1 andformed in a manner to cut out part of the radially inner part of theannular flange 15 a. One end (front end) of the pivot shaft 17, aboutwhich the third lens group frame 16 is pivoted, is supported by a shaftbearing hole 15 i (see FIG. 16) formed on the third lens group movingring 15, and the other end (rear end) of the pivot shaft 17 is supportedby a shaft support member 24 (see FIG. 5). The shaft support member 24is fixed to the third lens group moving ring 15 by a set screw 25.

As shown in FIG. 14, the third lens group frame 16 is provided with acylindrical lens holder 16 b, a swing arm 16 c, a pivoted cylindricalportion 16 d, and an engaging protrusion 16 e. The cylindrical lensholder 16 b holds the third lens group LG3. The swing arm 16 c extendsin a radial direction of the cylindrical lens holder 16 b and connectsto the pivoted cylindrical portion 16 d. The engaging protrusion 16 eextends from the cylindrical lens holder 16 b in a direction differentfrom the direction of extension of the swing arm 16 c. The pivotedcylindrical portion 16 d is provided with a bearing hole (through-hole)which extends in a direction parallel to the optical axis of the thirdlens group LG3. The pivot shaft 17 is inserted into this bearing hole.The third lens group frame 16 is provided in the vicinity of the pivotedcylindrical portion 16 d with a cam engaging projection 16 f which ispositioned eccentrically with respect to the axis of the pivot shaft 17,and the aforementioned cam surface 16 a is formed at the rear end of thecam engaging projection 16 f. The third lens group frame 16 is supportedby the third lens group moving ring 15 therein so that the major portionof the cylindrical lens holder 16 b is positioned in the spaceimmediately behind the middle flange 15 e, and only the front end of thecylindrical lens holder 16 b is positioned (inserted) in the throughhole 15 f (see FIGS. 2 and 3).

According to the above described structure, the third lens group frame16 can rotate about the pivot shaft 17 in a predetermined range ofrotation relative to the third lens group moving ring 15. Morespecifically, the rotational range of the third lens group frame 16ranges between an upward movement limit (see FIGS. 17, 19 and 20) atwhich the engaging protrusion 16 e comes in contact with the rotationlimit pin 15 g and a downward movement limit (see FIGS. 18, 21 and 22)positioned below the upward movement limit. Since the pivot shaft 17extends parallel to the imaging optical axis Z1, the third lens groupLG3 moves in the space immediately behind the middle flange 15 e by arotation of the third lens group frame 16 with the axis of the thirdlens group LG3 maintaining parallel to the imaging optical axis Z1.

As shown in FIG. 15, the position-control cam bar 21 a is formed on theimage sensor holding unit 21 to project forward at a position notinterfering with the AF lens frame 51. When the AF lens frame 51 movesto the rear movement limit (rearmost position) thereof, the front end ofthe position-control cam bar 21 a projects further forward from the AFlens frame 51. The retracting cam surface 21 b having a predetermineddegree of inclination relative to the imaging optical axis Z1 is formedat the front end of the position-control cam bar 21 a as describedabove, and the position-control cam bar 21 a is provided along a sideedge thereof with a displaced-position holding surface 21 c whichextends rearward from the retracting cam surface 21 b in a directionparallel to the imaging optical axis Z1.

Operations of the third lens group LG3 and other associated elements,which are supported by the above described accommodating structure, willbe hereinafter discussed. As described above, the position of the thirdlens group moving ring 15 with respect to the image sensor holding unit21 in the optical axis direction is determined by a combination of theforward/rearward movement of the third lens group moving ring 15 by thecam diagrams of the plurality of third-lens-group guide cam grooves 18 eand the forward/rearward movement of the helicoid ring 18 itself.However, in the retracted state of the zoom lens 5, the plurality ofthird-lens-group-control cam followers 15 c of the third lens groupmoving ring 15 are positioned in the circumferential groove portions 18e 2 of the plurality of third-lens-group guide cam grooves 18 e,respectively, so that the plurality of third-lens-group-control camfollowers 15 c move forward, integrally with the helicoid ring 18 in theoptical axis direction, without moving relative to the helicoid ring 18in the optical axis direction, even if the helicoid ring 18 rotates in alens barrel advancing direction. Thereafter, when the helicoid ring 18comes into the aforementioned fixed-position rotational state, in whichthe helicoid ring 18 rotates without changing the position thereof inthe optical axis direction, the plurality of third-lens-group-controlcam followers 15 c are guided by the plurality of third-lens-group guidecam grooves 18 e of the helicoid ring 18 to move further forward. Inshort, the third lens group moving ring 15 is positioned in a frontposition where the third lens group moving ring 15 is at a sufficientdistance forward from the image sensor holding unit 21 when the zoomlens 5 is set at or near the telephoto extremity as shown in FIG. 3; thethird lens group moving ring 15 is positioned closer to the image sensorholding unit 21 than when the zoom lens 5 is set at the telephotoextremity but still positioned some distance away from the image sensorholding unit 21 when the zoom lens 5 is set at the wide-angle extremityas shown in FIG. 2; and the third lens group moving ring 15 ispositioned closest to the image sensor holding unit 21 when the zoomlens 5 is in the retracted state as shown in FIG. 1. The third lensgroup LG3 is displaced from (radially retracted away from) the imagingoptical axis Z1 by utilizing the retracting rearward movement of thethird lens group moving frame 15 from a position in the zooming range(specifically the wide-angle extremity position) to the radiallyretracted position (rearmost position).

In the zooming range between the wide-angle extremity and the telephotoextremity, the third lens group frame 16 is held still at a fixedposition by the engagement of the end of the engaging protrusion 16 ewith the rotation limit pin 15 g (see FIGS. 19 and 20). In this state,the optical axis of the third lens group LG3 is coincident with theimaging optical axis Z1 as shown in FIGS. 2 and 3.

Upon the main switch of the digital camera being turned OFF in theready-to-photograph state of the zoom lens 5, the AF motor 160 is drivenin the lens barrel retracting direction to move the AF lens frame 51rearward, toward the image sensor holding unit 21 to the rear movementlimit (retracted position) shown in FIG. 15. At this time, the spacebetween the low-pass filter LF (and the image sensor 60), which issupported by the image sensor holding unit 21, and the fourth lens groupLG4, which is supported by the AF lens frame 51, is reduced. Inaddition, in a state where the AF lens frame 51 is in the rear movementlimit, the front end of the position-control cam bar 21 a is positioned(projects) in front of the AF lens frame 51 in the optical axisdirection.

Subsequently, the zoom motor 150 is driven in the lens barrel retractingdirection to perform the above described lens barrel retractingoperation. When the zoom lens 5 is set at the wide-angle extremity asshown in FIG. 2, the plurality of third-lens-group-control cam followers15 c of the third lens group moving ring 15 are positioned out of themovement-control groove portions 18 e 1 of the plurality ofthird-lens-group guide cam grooves 18 e and positioned in thecircumferential groove portions 18 e 2, respectively. Upon the zoommotor 150 continuing to drive in the lens barrel retracting directionbeyond the wide-angle extremity of the zoom lens 5, the third lens groupmoving ring 15 moves rearward with the helicoid ring 18 in the opticalaxis direction, thus approaching the image sensor holding unit 21. Thethird lens group frame 16 moves rearward with the third lens groupmoving ring 15, and thereafter, the cam surface 16 a comes into contactwith the retracting cam surface 21 b of the position-control cam bar 21a. The cam surface 16 a and the retracting cam surface 21 b are formedas lead surfaces which are shaped to generate a component force whichrotates the third lens group frame 16 about the pivot shaft 17counterclockwise with respect to FIGS. 19 and 20, as the cam surface 16a and the retracting cam surface 21 b slide against each other in theoptical axis direction. Therefore, a further rearward movement of thethird lens group frame 16 (together with the third lens group movingring 15) with the cam surface 16 a and the retracting cam surface 21 bremaining in contact with each other causes the third lens group frame16 to rotate against the biasing force of the torsion spring 39 in adirection to move the engaging protrusion 16 e away from the rotationlimit pin 15 g (i.e., in a direction to move the cylindrical lens holder16 b downward) as shown in FIG. 21.

As shown in FIG. 22, when the third lens group frame 16 rotates to aposition corresponding to the off-axis displaced position of the thirdlens group LG3, the cam engaging projection 16 f rises over theretracting cam surface 21 b to be engaged with the displaced-positionholding surface 21 c. Since the displaced-position holding surface 21 clies in a plane parallel to the imaging optical axis Z1, a componentforce in a direction to further rotate the third lens group frame 16toward the off-axis displaced position no longer acts on the third lensgroup frame 16. The displaced-position holding surface 21 c prevents thethird lens group frame 16 from returning to the on-axis position by thebiasing force of the torsion spring 39, thus holding the third lensgroup frame 16 in the off-axis displaced position.

As shown in FIG. 1, the cylindrical lens holder 16 b of the third lensgroup frame 16, which is moved to the off-axis displaced position,enters the accommodation space 15 h (i.e., passes through thethrough-hole portion 15 h 1 and enters the semi-circular recessedportion 15 h 2), and projects radially outwards, beyond the cylindricalportion 15 d. The second lens group holding ring 8 and the second linearguide ring 10, the cam ring 11, the first lens group holding ring 19,and the frontmost external barrel 12 are arranged around the third lensgroup moving ring 15, in that order in a radial direction from theimaging optical axis Z1. The outer edge of the cylindrical lens holder16 b of the third lens group frame 16 in the off-axis displaced positionreaches a radial position that overlaps the frontmost external barrel 12as viewed from the front. To prevent the third lens group frame 16positioned in the off-axis displaced position from interfering with eachof these members of the zoom lens 5 (e.g., the second lens group holdingring 8, the second linear guide ring 10, the cam ring 11, the first lensgroup holding ring 19, and the frontmost external barrel 12, etc.), astructure which will be discussed hereinafter is adopted.

The second lens group holding ring 8 is provided in the vicinity of therear end thereof with a cutout 8 d (part of which is shown in FIG. 5)which is formed through the second lens group holding ring 8 in a radialdirection thereof. The second linear guide ring 10 is provided on theinner edge of the rear end flange 10 a with a semi-circular recessedportion 10 e (see FIG. 5). In addition, the first lens group holdingring 19 is provided, in the vicinity of the rear end thereof which ispositioned aside from the plurality of third-lens-group-control camfollowers 15 c, with a cutout 19 d (see FIGS. 6 and 12). In addition,the frontmost external barrel 12 is provided, on an inner peripherythereof in the vicinity of the rear end of the frontmost external barrel12, with a semi-circular recessed portion 12 d (see FIG. 6). The cutouts8 d and 19 d are positioned to be radially aligned with the through-holeportion 15 h 1 of the third lens group moving ring 15 (so as to becommunicatively connected to the through-hole portion 15 h 1 in a radialdirection) in the retracted state of the zoom lens 5, and the sizes andshapes of the cutouts 8 d and 19 d are determined so as to allow thecylindrical lens holder 16 b to be inserted therethrough (projectradially outwards through the cutouts 8 d and 19 d). The semi-circularrecessed portions 10 e and 12 d are positioned to align with thesemi-circular recessed portion 15 h 2 of the third lens group movingring 15 as viewed from front (i.e., in the optical axis direction), andthe sizes and shapes of the semi-circular recessed portions 10 e and 12d are determined to allow the cylindrical lens holder 16 b of the thirdlens group frame 16 to be inserted therein.

As shown in FIGS. 17 and 18, the cam ring 11 is provided in front of therear end flange 11 c and the small-diameter flange 11 d with acylindrical portion 11 g. The set of three second-lens-group guide camgrooves 11 b are formed on an inner peripheral surface of thecylindrical portion 11 g, and the set of three first lead cam grooves 11e and the three pairs of second lead cam grooves 11 f if are formed onan outer peripheral surface of the cylindrical portion 11 g. The camring 11 is provided with an accommodation space 11 h which consists of athrough-hole portion 11 h 1 and a semi-circular recessed portion 11 h 2.The through-hole portion 11 h 1 is formed through the cylindricalportion 11 g in a radial direction on a portion thereof which does notoverlap any of the cam grooves 11 b, 11 e and 11 f. The semi-circularrecessed portion 11 h 2 is communicatively connected to the through-holeportion 11 h 1 and formed in a manner to cutout part of the radiallyinner portions of the rear end flange 11 c and the small-diameter flange11 d. The through-hole portion 11 h 1 is positioned to be radiallyaligned with the through-hole portion 15 h 1 of the third lens groupmoving ring 15 in the retracted state of the zoom lens 5, and thesemi-circular recessed portion 11 h 2 is positioned to align with thesemi-circular recessed portion 15 h 2 of the third lens group movingring 15 as viewed from front (i.e., in the optical axis direction). Thesizes and shapes of the through-hole portion 11 h 1 and thesemi-circular recessed portion 11 h 2 are determined to allow thecylindrical lens holder 16 b of the third lens group frame 16 to beinserted therein.

Due to this structure, in the retracted state of the zoom lens 5, thecylindrical lens holder 16 b of the third lens group frame 16 entersinto the accommodation spaces 11 h and 15 h of the cam ring 11 and thethird lens group moving ring 15 (see FIGS. 18 and 22 for reference tothe entering operation of the cylindrical lens holder 16 b into theaccommodation space 11 h of the cam ring 11), while the cylindrical lensholder 16 b of the third lens group frame 16 enters into the cutoutportions 8 d and 19 d and the semicircular recessed portions 10 e and 12d of the second lens group holding ring 8, the second linear guide ring10, the first lens group holding ring 19, the cam ring 11 and thefrontmost external barrel 12 (see FIG. 1). The zoom lens 5 can bereduced in diameter by a greater degree than a zoom lens that is notequipped with these relief structures (cutout portions) for preventionof interference. As can be understood from FIG. 1, in the retractedstate of the zoom lens 1, the cylindrical lens holder 16 b is positionedoutside the fourth lens group LG4 and the holding frame of the imagesensor 60, so that the cylindrical lens holder 16 b cannot be broughtcloser to the imaging optical axis Z1 any further. Therefore, supposingthat the zoom lens 5 is not provided, on the third lens group movingring 15 and other cylindrical members positioned radially outside thethird lens group moving ring 15, with any of the above describedaccommodation spaces, cutouts and semicircular recessed portions, eachof these cylindrical members of the zoom lens 5 would need to be madelarger in inner diameter to be prevented from interfering with thecylindrical lens holder 16 b positioned in the off-axis displacedposition. In contrast, in the present embodiment of the zoom lens 5, thecylindrical lens holder 16 b can be retracted (displaced) to a radialposition to overlap the frontmost external barrel 12 as viewed fromfront with no increase in diameter of the cylindrical members of thezoom lens 5. Consequently, the retracted state (fully-retracted state)of the zoom lens 5 that has superior space utilization efficiency isachieved, which makes it possible to achieve miniaturization of thewhole zoom lens 5.

Each of the second lens group holding ring 8, the second linear guidering 10, the first lens group holding ring 19 and the frontmost externalbarrel 12 is a linearly movable member which is guided linearly in theoptical axis direction, similar to the third lens group moving ring 15.Accordingly, the formation positions of the cutouts 8 d and 19 d and thesemicircular recessed portions 10 e and 12 d only need to be determinedso as to overlay the accommodation space 15 h as viewed from front inthe retracted state of the zoom lens 5. On the other hand, unlike theselinearly movable members, the cam ring 11 is a rotatable member, andaccordingly, the positions of the accommodation space 11 h on the camring 11 and the accommodation space 15 h on the third lens group movingring 15 in the rotating direction about the imaging optical axis Z1 needto be reliably coincident with each other to prevent the cam ring 11from interfering with the third lens group frame 16 when the cylindricallens holder 16 b enters the accommodation space 11 h and when thecylindrical lens holder 16 b exits from the accommodation space 11 h.The present embodiment of the zoom lens 5 is provided with an idlemechanism for preventing the cam ring 11 from rotating for apredetermined period of time at the beginning of rotation of thehelicoid ring 18 even if the helicoid ring 18 rotates when the zoom lens5 performs the lens barrel advancing direction from the retracted state.Although briefly described above, this idle mechanism is configured suchthat the set of three guide rollers 26 that are fixed to the cam ring 11are engaged in the circumferential groove portions 18 f 2 of the set ofthree roller-engaging grooves 18 f, respectively, so that rotation ofthe helicoid ring 18 is not transferred to the cam ring 11 via the setof three guide rollers 26 when the zoom lens 5 is in the retractedstate.

FIGS. 23 through 27 show operations of the idle mechanism for the camring 11. “R2”, “W2” and “T2” in the roller-guiding cam slot 14 e shownin FIGS. 23 through 27 represent the positions of each guide roller 26relative to the associated roller-guiding cam slot 14 e when the zoomlens 5 is in the retracted state, set at the wide-angle extremity andset at the telephoto extremity, respectively. FIG. 23 shows a state whenthe zoom lens 5 is in the retracted state. In this state, each guideroller 26 is in the vicinity of one of the circumferentially oppositeends (the left end with respect to FIG. 23) of the circumferentialgroove portion 18 f 2 which is farther from the joint between therotational transfer groove portion 18 f 1 and the circumferential grooveportion 18 f 2. In addition, each guide roller 26 is positioned at aretraction position R2 in the vicinity of the rear end of the associatedroller-guiding cam slot 14 e of the first linear guide ring 14. Theentire retraction range between the wide-angle extremity position W2 andthe retraction position R2 of each roller-guiding cam slot 14 e isformed as an inclined slot which includes an axial-direction componentin the axial direction of the first linear guide ring 14 and which isinclined with respect to both the axial direction and thecircumferential direction of the first linear guide ring 14. Note thatthe term “includes an axial-direction component” refers to a profile inwhich the positional change within, e.g., a slot/groove includes apositional change in the axial direction”. Accordingly, when each guideroller 26 is positioned in the retraction position R2 of eachroller-guiding cam slot 14 e, the position of each guide roller 26 inthe optical axis direction is determined by the axially-opposed wallsurfaces (the front and rear wall surfaces in the optical axisdirection) in the circumferential groove 18 f 2 of the associatedroller-engaging groove 18 f. More specifically, each guide roller 26 ispressed against the front wall surface in the circumferential groove 18f 2 of the associated roller-engaging groove 18 f by the above describedbiasing structure using the inter-lens-group biasing spring 34, and theposition of the cam ring 11 in the optical axis direction is determinedby this front wall surface as a reference surface. In addition to thisfront wall surface, the rear wall surface is also formed over the entirerange of the circumferential groove 18 f 2 of each roller-engaginggroove 18 f, and accordingly, each guide roller 26 is prevented fromboth moving forward and moving rearward. In this retracted state of thezoom lens 5, the third lens group LG3 is held at the off-axis displacedposition while the cylindrical lens holder 16 b of the third lens groupframe 16 is positioned in the accommodation space 11 h of the cam ring11 as shown in FIGS. 18 and 22.

When the helicoid ring 18 rotates in the lens barrel advancing directionfrom the retracted state shown in FIG. 23, the position of each guideroller 26 in the circumferential groove 18 f 2 of the associatedroller-engaging groove 18 f varies, and in a short time, each guideroller 26 reaches a point in the vicinity of the joint between therotational transfer groove portion 18 f 1 and the circumferential grooveportion 18 f 2 as shown in FIG. 24. On the other hand, the position ofeach guide roller 26 relative to the associated roller-guiding cam slot14 e does not yet vary, and accordingly, each guide roller 26 remainsheld at the retraction position R2. In the operational section of theidle mechanism from the state shown in FIG. 23 to the state shown inFIG. 24, the rotational force of the helicoid ring 18 is not transferredto the set of three guide rollers 26, so that the cam ring 11 islinearly advanced forward in the optical axis direction without rotatingtogether with the helicoid ring 18 which is advanced while rotating dueto the engagement of the outer helicoid 18 a with the inner helicoid 22a. Additionally, in the operational section of the idle mechanism fromthe state shown in FIG. 23 to the state shown in FIG. 24, the pluralityof third-lens-group-control cam followers 15 c of the third lens groupmoving ring 15 are engaged in the circumferential groove portions 18 e 2of the plurality of third-lens-group guide cam grooves 18 e,respectively, so that the third lens group moving ring 15 is advancedwith the helicoid ring 18 and the cam ring 11 in the optical axisdirection. Thereafter, upon the third lens group moving ring 15 beingadvanced by a predetermined amount, the cam engaging projection 16 f ofthe third lens group frame 16 is disengaged from the position-controlcam bar 21 a, and the third lens group frame 16 starts rotating in adirection to move the third lens group LG3 to the on-axis position bythe biasing force of the torsion spring 39 as shown in FIG. 21. As canbe understood by a comparison between FIGS. 21 and 22, the cam ring 11is not rotated until the cylindrical lens holder 16 b of the third lensgroup frame 16 is disengaged (completely withdrawn) from theaccommodation space 11 h, so that no interference occurs between the camring 11 and the cylindrical lens holder 16 b.

A further rotation of the helicoid ring 18 in the lens barrel advancingdirection causes each guide roller 26 to reach the boundary between therotational transfer groove portion 18 f 1 and the circumferential grooveportion 18 f 2 as shown in FIG. 25. Thereupon, one of thecircumferentially-opposed side wall surfaces (the right side wallsurface with respect to FIG. 25) of the rotational transfer grooveportion 18 f 1 can transfer rotation of the helicoid ring 18 in the lensbarrel advancing direction to the set of three guide rollers 26.Thereafter, the cam ring 11 starts rotating, and each guide roller 26moves away from the retraction position R2 of the associatedroller-guiding cam slot 14 e to move forward (i.e., toward thewide-angle extremity position W2) as shown in FIG. 26. Due to aninclined cam track of each roller-guiding cam slot 14 e, each guideroller 26 is moved (moved forward) into the rotational transfer grooveportion 18 f 1 of the associated roller-engaging groove 18 f.Thereafter, this engaged state in which each guide roller 26 is engagedwith the rotational transfer groove portion 18 f 1 of the associatedroller-engaging groove 18 f is maintained, so that the cam ring 11rotates with the helicoid ring 18 whenever the helicoid ring 18 rotatesuntil the lens barrel retracting operation is again performed. Since thethird lens group frame 16 is already positioned away from theaccommodation space 11 h and moved to the on-axis position uponcommencement of rotation of the cam ring 11 as shown in FIG. 20, the camring 11 and the third lens group frame 16 do not interfere with eachother.

FIG. 27 shows the relative position among one of the threeroller-engaging grooves 18 f, the associated roller-guiding cam slot 14e and the associated guide roller 26 in a state where the zoom lens 5 isadvanced to the wide-angle extremity. In this state, as shown in FIG.19, the cam ring 11 has been rotated by a predetermined angle ofrotation from the angular position for retraction (see FIGS. 21 and 22)where the cam ring 11 is prevented from rotating. In addition, the camring 11 has been advanced relative to the helicoid ring 18 by beingguided by the set of three roller-guiding cam slots 14 e. Furtherrotating the helicoid ring 18 in the lens barrel advancing directioncauses each guide roller 26 to be guided by the associatedroller-guiding cam slot 14 e in the zooming range thereof from thewide-angle extremity position W2 to the telephoto extremity position T2,thus causing the cam ring 11 to also move forward and rearward in theoptical axis direction while rotating.

In the lens barrel retracting operation of the zoom lens 5, the cam ring11 operates in the reverse manner to the above described advancingoperation. Upon each guide roller 26 being guided by the associatedroller-guiding cam slot 14 e therein to a position at the vicinity ofthe retraction position R2, each guide roller 26 reaches the rear end ofthe rotational transfer groove portion 18 f 1 of the associatedroller-engaging groove 18 f (see FIG. 25). Thereupon, each guide roller26 enters the circumferential groove portion 18 f 2 of the associatedroller-engaging groove 18 f in accordance with a further rotation of thehelicoid ring 18 in the lens barrel retracting direction (see FIG. 24).Thereafter, the cam ring 11 moves rearward with the helicoid ring 18without rotating while being subjected to optical-axis-directionposition control exercised by the circumferential groove portions 18 f 2of the set of three roller-engaging grooves 18 f (see FIG. 23). Thethird lens group frame 16 starts rotating from the on-axis positiontoward the off-axis displaced position by the position-control cam bar21 a at a timing (see FIG. 20) immediately before the cam ring 11 stopsrotating, but does not yet reach the off-axis displaced position at themoment the cam ring 11 stops rotating (see FIG. 21). The installationangle of the cam ring 11 is set so that the circumferential positions ofthe accommodation space 11 h of the cam ring 11 and the accommodationspace 15 h of the third lens group moving ring 15 coincide with eachother when the cam ring 11 reaches the rotation stop state (positionshown in FIG. 21). Therefore, when the cylindrical lens holder 16 benters the accommodation space 11 h as shown in FIG. 22, the state wherethe accommodation space 11 h of the cam ring 11 and the accommodationspace 15 h of the third lens group moving ring 15 are communicativelyconnected to each other is maintained, so that there is no possibilityof the cylindrical lens holder 16 b and the cam ring 11 interfering witheach other.

On the condition that rotation of the cam ring 11 has stopped at leastwhen the cylindrical lens holder 16 b enters the accommodation space 11h, the timing of stopping rotation of the cam ring 11 in the retractingoperation of the zoom lens 5 and the timing of commencement of theretracting (displacing) operation of the third lens group frame 16 tothe off-axis displaced position can be freely determined.

In this manner, the zoom lens 5 is equipped with the idle mechanism forpreventing the cam ring 11 from rotating for a predetermined period oftime at the final phase of the lens barrel retracting operation, inwhich the cylindrical lens holder 16 b rotates from the on-axis positionto the off-axis displaced position, regardless of the rotation of thehelicoid ring 18. Accordingly, when the third lens group frame 16 in theretracting (displacing) operation partly enters the accommodation space11 h of the cam ring 11, no interference occurs between the third lensgroup frame 16 and the cam ring 11 to thereby prevent a breakdown in thezoom lens 5. Specifically, in the present embodiment of the zoom lens,the timing and the position at which the cam ring 11 comes into arotation stop state are not easily misaligned (i.e., a discrepancy doesnot easily occur), which achieves a high-precision operation of the zoomlens 5.

As a comparison with the present embodiment of the idle mechanism forthe cam ring 11 that is incorporated in the zoom lens 5, the outline ofa known idle mechanism for a cam ring which is adopted in United StatesPatent Application Publication No. 2006/0034604 A1 (Japanese UnexaminedPatent Publication No. 2006-53444) will be hereinafter discussed withreference to FIGS. 35 and 36. This comparative example of the idlemechanism is different from the idle mechanism of the zoom lens 5 inthat a portion of each roller-guiding cam groove 14 e′ formed on alinear guide ring 14′ in the vicinity of the retraction position thereofis formed as a circumferential groove portion R2′ which is elongated ina direction orthogonal to the axis of the linear guide ring 14′, andthat the rear end of a circumferential groove portion 18 f 2′ (which iscontinuous with a rotational transfer groove portion 18 f 1′) of eachroller-engaging groove 18 f formed on a helicoid ring 18′ is open at therear end of the helicoid ring 18′. Upon a lens barrel retractingoperation being performed from a ready-to-photograph state, each guideroller 26′ is moved rearward in the rotational transfer groove portion18 f 1′ of the associated roller-engaging groove 18 f in accordance withrotation of the helicoid ring 18′ while being guided by the associatedroller-guiding cam groove 14 e′. Upon each guide roller 26′ exiting fromthe rear end of the rotational transfer groove portion 18 f 1′ of theassociated roller-engaging groove 18 f as shown in FIG. 35, rotation ofthe helicoid ring 18′ in the lens barrel retracting direction stopsbeing transferred to each guide roller 26′, so that the cam ring (notshown) having each guide roller 26′ stops rotating. Furthermore, as thehelicoid ring 18′ rotates in the lens barrel retracting direction, theposition of each guide roller 26′ in the circumferential groove portion18 f 2′ of the associated roller-engaging groove 18 f varies relative tothis circumferential groove portion 18 f 2′ so that each guide roller26′ abuts against a circumferential end (the right end with respect toFIG. 36) of the circumferential groove portion 18 f 2′ of the associatedroller-engaging groove 18 f as shown in FIG. 36. Upon completion of thelens barrel retracting operation, this circumferential end of thecircumferential groove portion 18 f 2′ presses the associated guideroller 26′ slightly leftward with respect to FIG. 36, which determinesthe final angular position of each guide roller 26′, i.e., the finalangular position of the cam ring (not shown) for retraction thereof.

In the structure of this comparative example, during the time from thestate shown in FIG. 35, in which the transmission of rotational force inthe lens barrel retracting direction from the helicoid ring 18′ to eachguide roller 26′ is canceled, to the state shown in FIG. 36, in whichthe lens barrel retracting direction is completed, the position of thecam ring having each guide roller 26′ in the rotating direction is notpositively controlled by either the helicoid ring 18′ (thecircumferential groove portion 18 f 2′ of the roller-engaging groove 18f) or the linear guide ring 14′ (the circumferential groove portion R2′of the roller-guiding cam groove 14 e′). Since the transmission ofrotational force by the helicoid ring 18′ has been canceled, the camring is not rotated; however, there is still room for variation inposition of the cam ring in the rotating direction thereof, and theprecise positioning of the cam ring in the rotating direction thereof isnot performed until the circumferential end (the right end with respectto FIG. 36) of the circumferential groove portion 18 f 2′ comes intoabutment against the associated guide roller 26′. Therefore, if it isattempted to provide the cam ring with an accommodation space similar tothe accommodation space 11 h of the cam ring 11, this accommodationspace needs to be relatively large taking into account of a possiblediscrepancy in the stop position of the cam ring. To miniaturize thelens barrel, it is desirable that this kind of accommodation space bemade as small as possible. In addition, if it is attempted to controlthe opening/shutting operation of the lens barrier mechanism by rotationof the cam ring in a similar manner to the above described embodiment,the lens barrel needs to be structured to have a certain amount ofmargin so that no error in operation of the lens barrier is caused byany discrepancy in the stop position of the cam ring. In the case ofpursuing miniaturization, operation accuracy and response of the lensbarrel, it is desirable that this sort of safety margin be as small aspossible. Namely, in an idle mechanism which allows no rotation to betransferred to a cam ring in the vicinity of the lens barrel retractingposition thereof, it is desirable that the idle mechanism be structurednot only to simply cancel the transmission of rotational force to thecam ring in a similar manner to the above described comparative examplebut also to precisely determine the position of the cam ring in therotating direction thereof at the time the transmission of rotationalforce to the cam ring is cancelled.

As described above, in the present embodiment of the zoom lens 5, eachguide roller 26 is engaged with the associated roller-guiding cam groove14 e at the retraction position R2 during the time the transmission ofrotational force from the helicoid ring 18 to the cam ring 11 iscanceled with the set of three guide rollers 26 being engaged in thecircumferential groove portions 18 f 2 of the set of threeroller-engaging grooves 18 f, respectively (see FIGS. 23 and 24). Unlikethe circumferential groove portion R2′ of each roller-guiding cam groove14 e′, the retraction position R2 of each roller-guiding cam groove 14 eis formed as an inclined groove portion which is inclined with respectto both the axial direction and the circumferential direction of thefirst linear guide ring 14, so that the position of each guide roller 26in the rotating direction thereof can be precisely determined by sidesurfaces (opposed surfaces) of each roller-guiding cam groove 14 e. Inaddition, the position of each guide roller 26 in the associatedroller-guiding cam groove 14 e also does not vary since the set of threeguide rollers 26 are prevented from moving in the optical axis directionby engagement with the circumferential groove portions 18 f 2 of the setof three roller-engaging grooves 18 f, respectively. Accordingly, uponthe transmission of rotational force from the rotational transfer grooveportions 18 f 1 of the set of three roller-engaging grooves 18 f to theset of three guide rollers 26 being canceled, the position of the camring 11 in the rotating direction thereof and the position of the camring 11 in the optical axis direction are securely precisely controlled,so that the cam ring 11 does not come into an unstable state in thevicinity of the retraction position thereof. Consequently, the size ofthe accommodation space 11 h can be reduced to a minimum; moreover, asafety margin secured between the cam ring 11 and the lens barriermechanism can be minimized, which makes it possible to achieve furtherminiaturization of the zoom lens 5 and an improvement in operationaccuracy of the zoom lens 5.

Another feature of the zoom lens 5 on the association between the firstlens group LG1 and the lens barrier mechanism will be discussedhereinafter. As a precondition for this type of lens barrier mechanism,the barrier blades of the lens barrier mechanism must be located at aposition in the optical axis direction where the barrier blades do notinterfere with the frontmost lens group (the first lens group LG1 in thepresent embodiment of the zoom lens) when the barrier blades are shut.In actuality, in the present embodiment of the zoom lens 5, both thepair of inner barrier blades 45 and the pair of outer barrier blades 46are moved into the space in front of the first lens group LG1 and do notinterfere with the first lens group LG1 in the retracted state of thezoom lens 5 as shown in FIGS. 1 and 32. On the other hand, wide-anglezoom lenses (zoom lenses with a wide-angle focal length range) are knownin the art which are configured so that the frontmost lens group isadvanced beyond a plane in which the barrier blades of a lens barriermechanism lie in the ready-to-photograph state to prevent rays of lightincident on the frontmost lens group from being intercepted by the inneredge of a photographing aperture of the lens barrier mechanism when thezoom lens is in the ready-to-photograph state. The present embodiment ofthe zoom lens 5 is such a type of zoom lens. As shown in FIGS. 2, 3 and33, the first lens group LG1 is advanced relative to the lens barriermechanism so that the front part of the first lens group LG1 ispositioned in front of a plane in which the photographing aperture 41 aof the barrier mount frame 41 lies, i.e., in which the pair of innerbarrier blades 45 and the pair of outer barrier blades 46 lie. Suchmovement of the first lens group LG1 relative to the lens barriermechanism is carried out by controlling the positions of the frontmostexternal barrel 12, which holds the lens barrier mechanism, and thefirst lens group holding frame 19, which holds the first lens group LG1.

FIG. 31 shows an enlarged view of a portion of the outer peripheralsurface of the cam ring 11 to illustrate the set of three first lead camgrooves 11 e and the three pairs of second lead cam grooves 11 f, whichare formed on the outer peripheral surface of the cam ring 11 and usedto control the relative position between the frontmost external barrel12 and the first lens group holding ring 19. In FIG. 31, “R3” designatesthe position (retraction position) of each lead projection 12 c in theassociated lead cam groove 11 e in the retracted state of the zoom lens5, and “R4” designates the position (retraction position) of eachfirst-lens-group-control cam follower 19 c in the associated second leadcam groove 11 f in the retracted state of the zoom lens 5. Similarly, inFIG. 31, “W3” designates the position of each lead projection 12 c inthe associated lead cam groove 11 e when the zoom lens 5 is set at thewide-angle extremity, “W4” designates the position of eachfirst-lens-group-control cam follower 19 c in the associated second leadcam groove 11 f when the zoom lens 5 is set at the wide-angle extremity,“T3” designates the position of each lead projection 12 c in theassociated lead cam groove 11 e when the zoom lens 5 is set at thetelephoto extremity, and “T4” designates the position of eachfirst-lens-group-control cam follower 19 c in the associated second leadcam groove 11 f when the zoom lens 5 is set at the telephoto extremity.

As can be understood from FIG. 31, each pair of second lead cam grooves11 f consists of a front lead cam groove 11 f-A and a rear cam groove 11f-B which are formed at different positions in the optical axisdirection. The rear lead cam groove 11 f-B does not have a rear endportion thereof at the retraction position R4 of the associatedfirst-lens-group-control cam follower 19 c, whereas the front lead camgroove 11 f-A has a rear end portion thereof which includes theretraction position R4. In contrast, the front lead cam groove 11 f-Adoes not have a front end portion thereof at the retraction position T4of the associated first-lens-group-control cam follower 19 c, whereasthe rear lead cam groove 11 f-B has a front end portion thereof whichincludes the retraction position T4. However, the groove-missing portionof one second lead cam groove 11 f of each pair of second lead camgrooves 11 f (11 f-A and 11 f-B) is complemented (compensated) by theother second lead cam grooves 11 f so that each pair offirst-lens-group-control cam followers 19 c is engaged with at least oneof the associated pair of second lead cam grooves 11 f at all times.Accordingly, the position control performed by the three pairs of secondlead cam grooves 11 f is maintained in either engaged state. Inaddition, although each pair of first-lens-group-control cam followers19 c (front and rear first-lens-group-control cam followers 19 c) areengaged in the front lead cam groove 11 f-A and the rear lead cam groove11 f-B in the vicinity of the wide-angle extremity position W4,respectively, the front lead cam groove 11 f-A is wider in groove widththan the rear lead cam groove 11 f-B in this range of engagement, andthe position control for each pair of first-lens-group-control camfollowers 19 c is accurately performed by the rear lead cam groove 11f-B of the associated pair of second lead cam grooves 11 f. Namely, inthe zooming range from the wide-angle extremity to the telephotoextremity, the rear lead cam groove 11 f-B of each pair of second leadcam grooves 11 f is precisely engaged with the rearfirst-lens-group-control cam follower 19 c of the associated pair offirst-lens-group-control cam followers 19 c to control the positionthereof. Conversely, in the retraction position, the front lead camgroove 11 f-A of each pair of second lead cam grooves 11 f is engagedwith the front first-lens-group-control cam follower 19 c of theassociated pair of first-lens-group-control cam followers 19 c tocontrol the position of each pair of first-lens-group-control camfollowers 19 c. In other words, the rear lead cam grooves 11 f-B of thethree pairs of second lead cam grooves 11 f serve as lead cam groovesused for the zooming range, while the front lead cam grooves 11 f-A ofthe three pairs of second lead cam grooves 11 f serve as lead camgrooves used for the retraction position.

As described above, the three pairs of second lead cam grooves 11 f,each pair of which includes the front lead cam groove 11 f-A and therear lead cam groove 11 f-B, are formed as lead grooves shaped intostraight cam grooves in developed plan view as shown in FIGS. 10 and 31.Each of the three first lead cam grooves 11 e, which is located at aposition between the front lead cam groove 11 f-A and the rear lead camgroove 11 f-B of the associated pair of second lead cam grooves 11 f inthe optical axis direction, has a crank-shaped cam track, including afront lead groove portion K1, a rear lead groove portion K2 and adifferential groove portion K3 via which the front lead groove portionK1 and the rear lead groove portion K2 are connected. The differentialgroove portion K3 extends in the rotating direction of the cam ring 11(circumferential direction orthogonal to the axis of the cam ring 11),thus including no axial-direction component in the axial direction ofthe cam ring 11.

Each of the three lead projections 12 c, which are respectively engagedin the set of three first lead cam grooves 11 e, is shaped into apolygonal prism which is provided with a pair of lead surfaces 12 c 1and a pair of orthogonal surfaces 12 c 2. The pair of lead surfaces 12 c1 are parallel to the direction of formation of the front lead grooveportion K1 and the rear lead groove portion K2, and the pair oforthogonal surfaces 12 c 2 are parallel to the differential grooveportion K3 (i.e., extend in a direction orthogonal to the axialdirection of the frontmost external barrel 12). The groove width of thefront lead groove portion K1 is a normal groove width which correspondsto the distance between the pair of lead surfaces 12 c 1 of theassociated lead projection 12 c, and the groove width of the rear leadgroove portion K2 is greater than this distance between the pair of leadsurfaces 12 c 1. The rear end of the rear lead groove portion K2 is openon a rear end surface of the cam ring 11, and this rear end opening isgreater in groove width than the remaining part of the rear lead grooveportion K2. Although the details will be discussed later, the front leadgroove portion K1 is precisely engaged with the associated leadprojection 12 c and serves as a zooming-range lead groove portion forprecisely controlling the position of the associated lead projection 12c in the zooming range from the wide-angle extremity to the telephotoextremity, and the rear lead groove portion K2 serves as a retractionlead groove portion for controlling the retraction position of theassociated lead projection 12 c.

In the retracted state of the zoom lens 5, each of the three leadprojections 12 c is positioned in the vicinity of the rear end openingof the rear lead groove portion K2 of the associated first lead camgroove 11 e. In FIG. 31, “D1” represents the distance in the opticalaxis direction between the lead projection 12 c positioned at theretraction position R3 and the rear first-lens-group-control camfollower 19 c positioned at the retraction position R4 and disengagedfrom the rear lead cam groove 11 f-B. In this state, the first lensgroup LG1, which is held by the first lens group holding ring 19, andthe pair of inner barrier blades 45 and the pair of outer barrier blades46, which are held by the frontmost external barrel 12, are positionedso as not to overlap each other in the optical axis direction as shownin FIG. 32, i.e., so as not to interfere with each other.

Upon the cam ring 11 moving from the above described irrotational state(idle state) to the interconnected rotational state, in which the camring 11 rotates with the helicoid ring 18, by an advancing operation ofthe zoom lens 5, each lead projection 12 c and eachfirst-lens-group-control cam follower 19 c move forward in theassociated first lead cam groove 11 e and the associated second lead camgroove 11 f, respectively. A movement of each lead projection 12 c inthe associated first lead cam groove 11 e for a predetermined amountwith the pair of lead surfaces 12 c 1 of the lead projection 12 c beingguided by the rear lead groove portion K2 of the associated first leadcam groove 11 e causes the lead projection 12 c to enter thedifferential groove portion K3 of the associated first lead cam groove11 e, so that the pair of orthogonal surfaces 12 c 2 of the leadprojection 12 c come in contact with axially-opposed wall surfaces ofthe differential groove portion K3 of the associated first lead camgroove 11 e, respectively. Thereupon, in accordance with the rotation ofthe cam ring 11 in the lens barrel advancing direction, the leadprojection 12 c is moved in the circumferential direction in thedifferential groove portion K3, so that the frontmost external barrel 12comes into a state where the frontmost external barrel 12 does not movein the optical axis direction relative to the cam ring 11. On the otherhand, since each first-lens-group-control cam followers 19 c continuesto be moved linearly along the linear cam track of the associated secondlead cam groove 11 f, the first lens group holding ring 19 is advancedin the optical axis direction relative to the frontmost external barrel12, namely, a difference in movement in the optical axis directionoccurs between the first lens group holding ring 19 and the frontmostexternal barrel 12. A further rotation of the cam ring 11 in the lensbarrel advancing direction causes each lead projection 12 c to move fromthe differential groove portion K3 of the associated first lead camgroove 11 e into the front lead groove portion K1 of the same first leadcam groove 11 e that is positioned in front of the differential grooveportion K3, so that each lead projection 12 c comes into a state whereit is guided with the pair of lead surfaces 12 c 1 thereof being guidedby the rear lead groove portion K2 of the associated first lead camgroove 11 e. The front lead groove portion K1 is narrower in groovewidth than the rear lead groove portion K2 so as to be capable ofguiding the associated lead projection 12 c more precisely than that inthe retracted state. Thereafter, upon each lead projection 12 c and eachfirst-lens-group-control cam follower 19 c reaching the wide-angleextremity position W3 and the wide-angle extremity position W4,respectively, the distance between the lead projection 12 c and the rearfirst-lens-group-control cam follower 19 c in the optical axis directionhas changed from the distance D2 to the distance D1. Namely, the firstlens group holding ring 19 has been moved forward relative to thefrontmost external barrel 12 by an amount of movement corresponding tothe difference between D1 and D2. Consequently, as shown in FIG. 33, thefirst lens group LG1 advances to apposition where the first lens groupLG1 overlaps the barrier blades 45 and 46 which are opened and thebarrier mount frame 41 in the optical axis direction, to thereby preventrays of light (object light) incident on the first lens group LG1 frombeing intercepted by the lens barrier mechanism. At the stage at whichthe frontmost external barrel 12 and the first lens group holding ring19 move relative to each other by the differential groove portions K3 ofthe set of three first lead cam grooves 11 e, the opening operations ofthe pair of inner barrier blades 45 and the pair of outer barrier blades46 of the lens barrier mechanism have been already completed, so thatthe first lens group LG1 does not interfere with any of the barrierblades 45 and 46 even if the first lens group LG1 is advanced to theposition shown in FIG. 33. In the zooming range from the wide-angleextremity positions W3 and W4 to the telephoto extremity positions T3and T4, the frontmost external barrel 12 and the first lens groupholding ring 19 are moved linearly in accordance with rotation of thecam ring 11, and the relative position between the first lens group LG1and the lens barrier mechanism does not change. In this zooming range,backlash between each lead projection 12 c and the associated first leadcam groove 11 e (the front lead groove portion K1 thereof) and backlashbetween each first-lens-group-control cam follower 19 c and theassociated second lead cam groove 11 f are eliminated by the biasingforce of the three first lens group biasing springs 23.

When the zoom lens 5 performs the lens barrel retracting operation, arelative movement between the frontmost external barrel 12 and the firstlens group holding ring 19 in a direction to retract (move rearward) thefirst lens group LG1 in the optical axis direction relative to the lensbarrier mechanism occurs while each lead projection 12 c moves in thedifferential groove portion K3 of the associated first lead cam groove11 e. Accordingly, as shown in FIG. 32, in the retracted state of thezoom lens 5, the front of the first lens group LG1 is shut (closed) bythe pair of inner barrier blades 45 and the pair of outer barrier blades46 so that the first lens group LG1 and barrier blades 45 and 46 do notinterfere with each other, and hence, the first lens group LG1 isprotected by the barrier blades 45 and 46.

As described above, by providing each first lead cam groove 11 e andeach second lead cam groove 11 f, which are formed on the outerperipheral surface of the cam ring 11, with different cam tracks fromeach other makes relative movement between the lens barrier mechanismand the first lens group LG1 possible. However, in optical design, themoving path of the first lens group LG1 is a nonlinear moving path M1(see FIG. 34), unlike the linear cam track of each second lead camgroove 11 f. It should be noted in FIG. 34 that the horizontal axisrepresents the angle of rotation of the cam ring 11 in the zoomingrange, and that the vertical axis represents the moving distance of thefirst lens group LG1 in the optical axis direction with the position ofthe first lens group LG1 at the wide-angle extremity being taken as areference position. A cam track M2 of each second lead cam groove 11 fis a linear cam track by which the amount of movement of the associatedfirst-lens-group-control cam follower 19 c per unit of rotation of thecam ring 11 is constant. Additionally, the movement of the cam ring 11itself is controlled so that the cam ring 11 moves in a nonlinear movingmanner in the optical axis direction so as to provide the nonlinearmoving path M1 to the first lens group LG1 while using the three pairsof second lead cam grooves 11 f that are linear in shape. Specifically,the zoom lens 5 is constructed to achieve the nonlinear moving path M1of the first lens group LG1 by combining a cam track M3 of eachroller-guiding cam slot 14 e of the first linear guide ring 14, which isengaged with the associated guide roller 26 of the cam ring 11, with thelinear cam track M2 of each second lead cam groove 11 f. As shown inFIG. 9, although each roller-guiding cam slot 14 e of the first linearguide ring 14 includes a linear lead shape in the range from theretraction position R2 to the wide-angle extremity position W2, thezooming range from the wide-angle extremity position W2 to the telephotoextremity T2 is non-linear corresponding to the cam track M3 shown inFIG. 34.

In this manner, when the movement of the first linear guide ring LG1 iscontrolled by the nonlinear moving path M1, excellent space utilizationefficiency and operation accuracy can be achieved by forming the threepairs of second lead cam grooves 11 f that are linear in shape on thecam ring 11 and also by making the set of three roller-guiding cam slots14 e of the first linear guide ring 14 contribute to (compensate for)the nonlinear component of the moving path M1. For instance, supposingthat each cam groove formed on the outer peripheral surface of the camring 11 is shaped so as to simply trace the actual nonlinear moving pathM1, unlike each second lead cam groove 11 f. This imaginary cam groove(M1) would be in the shape of a letter J which firstly extends rearwarda little in the optical axis direction from the wide-angle extremityposition and subsequently turns back to extend forward in the opticalaxis direction, and is required to solely give the first lens group LG1the amount of movement thereof for the entire zooming range, whichincreases the space in the optical axis direction which is required forthe formation of such an imaginary cam groove, thus increasing thelength of the cam ring in the optical axis direction. Furthermore,because this imaginary cam groove (M1) has a large inclination anglerelative to the rotation direction of the cam ring, the rotationalresistance of the cam ring when the cam ring guides cam followers isgreat. Additionally, in the case of attempting to form a plurality ofcam grooves (11 e and 11 f) having different capabilities on a singleperipheral surface of the cam ring like the cam ring 11 of the presentembodiment of the zoom lens 5, one cam groove would tend to interferewith another on the same peripheral surface of the cam ring if theplurality of cam grooves are nonlinear cam grooves such as the imaginarycam grooves (M1), so that it would be difficult to arrange differenttypes of cam grooves efficiently while achieving miniaturization of thecam ring.

In contrast, according to the cam mechanism in the present embodiment ofthe zoom lens 5, the above described problems can be overcome.Specifically, the amount of movement that each of the cam ring 11 andthe first linear guide ring 14 undertakes can be reduced because the camgrooves which give a predetermined amount of movement to the first lensgroup LG1 are divided and arranged separately on the cam ring 11 and thefirst linear guide ring 14 as the second lead cam grooves 11 f and theroller-guiding cam slots 14 e. As for the cam ring 11, the cam ring 11can be made smaller in size in the optical axis direction than the caseof forming the imaginary cam grooves (M1) that singly achieves anecessary moving path for the first lens group LG1. In addition, sincethe three pairs of second lead cam grooves 11 f, which are formed on thecam ring 11 as cam grooves for controlling the position of the firstlens group LG1, and the set of three roller-guiding cam slots 14 e,which are formed on the first linear guide ring 14, are smaller ininclination with respect to the rotation direction of the cam ring thanthe imaginary cam grooves (M1) that singly achieve a necessary movingpath for the first lens group LG1, each second lead cam groove 11 f andeach roller-guiding cam slot 14 e make it possible to reduce frictionalresistance and also make high-precision position control possible wheneach second lead cam groove 11 f and each roller-guiding cam slot 14 eguide the lead projections 12 c and the guide rollers 26, respectively.Additionally, since two types of cam grooves formed on the outerperipheral surface of the cam ring 11 are formed as the set of threefirst lead cam grooves 11 e and the three pairs of second lead camgrooves 11 f, respectively, both of which are linear in basic grooveshape, each first lead cam groove 11 e and the associated pair of secondlead cam grooves 11 f can be brought extremely close to each other.Moreover, an improvement in efficiency of space utilization on the camring 11 (a reduction of the area occupied by cam grooves on the camring) is also achieved. More specifically, in the set of three firstlead cam grooves 11 e and the three pairs of second lead cam grooves 11f, the front lead groove portion K1 of each first lead cam groove 11 econstitutes a zooming-range lead groove for precisely controlling theposition of the associated lead projection 12 c in the zooming rangefrom the wide-angle extremity to the telephoto extremity, while the rearlead cam groove 11 f-B of each pair of second lead cam grooves 11 fconstitutes a zooming-range lead groove for precisely controlling theposition of the associated pair of first-lens-group-control camfollowers 19 c in the zooming range from the wide-angle extremity to thetelephoto extremity. Additionally, in each of the three first lead camgrooves 11 e, the front lead groove portion K1 that serves as azooming-range lead groove is longer than the rear lead groove portion K2that serves as a retraction lead groove. Similarly, in each pair ofsecond lead cam grooves 11 f, the rear lead cam groove 11 f-B thatserves as a zooming-range lead groove is longer than the front lead camgroove 11 f-A that serves as a retraction lead groove. On the outerperipheral surface of the cam ring 11, these relatively long grooveportions, namely, the front lead groove portion K1 of each first leadcam groove 11 e and the rear lead cam groove 11 f-B of the associatedpair of second lead cam grooves 11 f are positioned closely to eachother. In other words, the front lead groove portion K1 of each firstlead cam groove 11 e is positioned closer to the rear lead cam groove 11f-B of the associated pair of second lead cam grooves 11 f than thefront lead cam groove 11 f-A of the associated pair of second lead camgrooves 11 f. With this arrangement, each first lead cam groove 11 e andeach pair of second lead cam grooves 11 f are arranged efficiently,without interfering with one another, in a limited peripheral space onthe cam ring 11.

In addition, in the set of three lead projections 12 c, since the pairof lead surfaces 12 c 1 and the pair of orthogonal surfaces 12 c 2 ofeach lead projection 12 c, each pair of which are parallel planesurfaces, are formed as contact surfaces against the opposed side wallsof the associated first lead cam groove 11 e, the contact area of eachlead projection 12 c which is in slide contact with the opposed sidewalls of the first lead cam groove 11 e is large, which makes itpossible to increase the strength against external force. It isdesirable that the frontmost external barrel 12 that has the set ofthree lead projections 12 c, in particular, be superior in strength inthis manner because the frontmost external barrel 12 serves as one ofthe elements of the zoom lens 5 which form the outward appearance of thezoom lens 5.

The specific structure of the above described embodiment of the zoomlens 5 is a merely example which embodies the present invention, so thatthe spirit and scope of the present invention are not limited by theabove described embodiment. For instance, although the cam mechanism inwhich backlash is eliminated by the inter-lens-group biasing spring 34is used as a drive mechanism for the second lens group LG2 and the thirdlens group LG3 in the above described embodiment of the zoom lens, thepresent invention can also be applied to a cam mechanism for otheroptical elements.

Although the biasing force of the inter-lens-group biasing spring 34acts on the second lens group LG2 and the third lens group LG3 in amanner to move the second lens group LG2 and the third lens group LG3away from each other in the above described embodiment, it is possiblethat the inter-lens-group biasing spring 34 be replaced by anotherbiasing device (e.g., extension spring) for biasing these two opticalelements in directions to move these two optical elements toward eachother. In this alternative case, in the reverse manner to the abovedescribed embodiment of the zoom lens, a rearward biasing force in theoptical axis direction acts on a front optical element (whichcorresponds to the second lens group LG2 of the above describedembodiment) and a first position control mechanism (cam mechanism) fordriving this front optical element. In addition, a first limit memberwhich is an element of this first position control mechanism is made tocome in contact with a forward-facing limit surface (which correspondsto the limit wall portion 22 f) of a reference barrel (which correspondsto the stationary barrel 22). Furthermore, in the reverse manner to theabove described embodiment of the zoom lens, a forward biasing force inthe optical axis direction acts on a rear optical element (whichcorresponds to the third lens group LG3 of the above describedembodiment) and a second position control mechanism (cam mechanism) fordriving this rear optical element. A second limit member which is anelement of this second position control mechanism is made to come incontact with a rearward-facing limit surface (which corresponds to thelimit wall portion 22 e) of the aforementioned reference barrel (whichcorresponds to the stationary barrel 22). Even in this configuration inwhich the biasing direction of the spring device is reversed in theaforementioned manner, a single spring device can eliminate backlash onthe entire system of two separate drive systems for controlling the twooptical elements, and effects similar to those obtained in the abovedescribed embodiment for structure simplification can be obtained.

Accordingly, either of the following two types of spring devices, i.e.,a type of spring device which biases the front optical element and therear optical element in directions away from each other, or another typeof spring device which biases the front optical element and the rearoptical element in directions toward each other, can be adopted so longas the spring device is of a type which biases the front optical elementand the rear optical element in opposite directions relative to eachother. When the former type of spring device (which biases the frontoptical element and the rear optical element in directions away fromeach other) is used, the forward biasing force which acts on the drivesystem (position control mechanism) for the front optical element isreceived by a rearward-facing limit surface of the reference barrel,while the rearward biasing force which acts on the drive system(position control mechanism) for the rear optical element is received bya forward-facing limit surface of the reference barrel. When the lattertype of spring device (which biases the front optical element and therear optical element in directions toward each other) is used, therearward biasing force which acts on the drive system (position controlmechanism) for the front optical element is received by a forward-facinglimit surface of the reference barrel, while the forward biasing forcewhich acts on the drive system (position control mechanism) for the rearoptical element is received by a rearward-facing limit surface of thereference barrel.

Obvious changes may be made in the specific embodiment of the presentinvention described herein, such modifications being within the spiritand scope of the invention claimed. It is indicated that all mattercontained herein is illustrative and does not limit the scope of thepresent invention.

1. A lens barrel comprising: an imaging optical system which includes afront optical element and a rear optical element which are relativelymovable in an optical axis direction; a spring installed between saidfront optical element and said rear optical element so as to increase adistance therebetween; a reference barrel which includes aforward-facing limit surface and a rearward-facing limit surface whichface forward and rearward in said optical axis direction, respectively;a first position control mechanism which includes a first rotationalmember and a first limit member, said first position control mechanismvarying a position of said front optical element in said optical axisdirection relative to said reference barrel in accordance with arotation of said first rotational member; and a second position controlmechanism which includes a second rotational member and a second limitmember, said second position control mechanism varying a position ofsaid rear optical element in said optical axis direction relative tosaid reference barrel in accordance with a rotation of said secondrotational member; wherein said first limit member is brought intocontact with said rearward-facing limit surface by a forward biasingforce of said spring to determine a position of said first positioncontrol mechanism while said second limit member is brought into contactwith said forward-facing limit surface by a rearward biasing force ofsaid spring to determine a position of said second position controlmechanism.
 2. The lens barrel according to claim 1, wherein saidforward-facing limit surface is formed at a position closer to the frontof said reference barrel in said optical axis direction than saidrearward-facing limit surface.
 3. The lens barrel according to claim 1,wherein said first position control mechanism comprises: a linear guidering serving as said first limit member, a position of which in saidoptical axis direction is determined by said rearward-facing limitsurface of said reference barrel, wherein said linear guide ring isprevented from rotating relative to said reference barrel; and a camring, a position of which in said optical axis direction is determinedby said linear guide ring and which determines said position of saidfront optical element in said optical axis direction, wherein saidsecond position control mechanism comprises: a rotating ring includingsaid second limit member, a position of which in said optical axisdirection is determined by said forward-facing limit surface of saidreference barrel and which rotates relative to said reference barrel;and at least one cam groove, formed on said rotating ring, fordetermining said position of said rear optical element in said opticalaxis direction.
 4. The lens barrel according to claim 3, wherein saidrearward-facing limit surface of said reference barrel is provided at anend of a linear guide groove for guiding said linear guide ring to movelinearly in said optical axis direction, and wherein said forward-facinglimit surface of said reference barrel is formed by a side wall of acircumferential groove for guiding said rotating ring so as to allowsaid rotating ring to rotate.
 5. The lens barrel according to claim 3,wherein said front optical element and said rear optical elementconstitute two lens groups of a zoom optical system, and wherein saidtwo lens groups are moved toward and away from each other in apredetermined moving manner in said optical axis direction by a rotationof said cam ring and said rotating ring when said zoom optical systemperforms a zooming operation.
 6. The lens barrel according to claim 1,wherein each of said front optical element and said rear optical elementis movable between a retracted position and a ready-to-photographposition in said optical axis direction, wherein said first limit memberand said second limit member are in contact with said rearward-facinglimit surface and said forward-facing limit surface, respectively, wheneach of said front optical element and said rear optical element is insaid ready-to-photograph position, and wherein said first limit memberand said second limit member are disengaged from said rearward-facinglimit surface and said forward-facing limit surface, respectively, wheneach of said front optical element and said rear optical element is insaid retracted position.
 7. The lens barrel according to claim 1,wherein said spring comprises a coil spring.
 8. The lens barrelaccording to claim 1, wherein said reference barrel comprises astationary member that is fixed to a camera body to which said lensbarrel is mounted.
 9. A lens barrel comprising: an imaging opticalsystem which includes a front optical element and a rear optical elementwhich are relatively movable in an optical axis direction; a springinstalled between said front optical element and said rear opticalelement so as to increase a distance therebetween; a reference barrelwhich includes a forward-facing limit surface and a rearward-facinglimit surface which face forward and rearward in said optical axisdirection, respectively; a first limit member for determining a positionin said optical axis direction of said front optical element; a secondlimit member for determining a position in said optical axis directionof said rear optical element; a first cam mechanism which is providedbetween said front optical element and said first limit member forcontrolling a position in said optical axis direction of said frontoptical element, said first cam mechanism transmitting a forward biasingforce of said spring to said first limit member so as to bring saidfirst limit member into contact with said rearward-facing limit surface;and a second cam mechanism which is provided between said rear opticalelement and said second limit member for controlling a position in saidoptical axis direction of said rear optical element, said second cammechanism transmitting a rearward biasing force of said spring to saidsecond limit member so as to bring said second limit member into contactwith said forward-facing limit surface.
 10. A lens barrel comprising: animaging optical system which includes a front optical element and a rearoptical element which are relatively movable in an optical axisdirection; a spring, installed between said front optical element andsaid rear optical element, for biasing said front optical element andsaid rear optical element in opposite directions one of toward and awayfrom each other in said optical axis direction; a reference barrel whichincludes a forward-facing limit surface and a rearward-facing limitsurface which face forwardly and rearwardly in said optical axisdirection, respectively; a first position control mechanism whichincludes a first rotational member and a first limit member, said firstposition control mechanism varying a position of said front opticalelement in said optical axis direction relative to said reference barrelin accordance with a rotation of said first rotational member; and asecond position control mechanism which includes a second rotationalmember and a second limit member, said second position control mechanismvarying a position of said rear optical element in said optical axisdirection relative to said reference barrel in accordance with arotation of said second rotational member; wherein said first limitmember is brought into contact with one of said forward-facing limitsurface and said rearward-facing limit surface which faces toward adirection opposite to a direction of a biasing force applied on saidfront optical element by said spring, and wherein said second limitmember is brought into contact with the other of said forward-facinglimit surface and said rearward-facing limit surface, which faces towarda direction opposite to a direction of a biasing force applied on saidrear optical element by said spring.